Introduce "Modern CMake" in FLTK

This is a big commit and there are too many changes to list them all.
The main changes are:

- rename all CMake build options to 'FLTK_*'
- export library targets with namespace (prefix) 'fltk::'
- standardize shared library target names with suffix '-shared'
- set public build properties on libraries for consumers
- document library names and aliases in README.CMake.txt
- document changes in "Migrating Code from FLTK 1.3 to 1.4"
- partial backwards compatibility for old user projects

Included but not directly related changes:

- fix Windows (Visual Studio) DLL build
- add CMake function fl_debug_target() to show target properties
- don't build test programs if FLTK is a subproject
- internal: reformat CMake code: remove space before '('

Thanks to Matthias and Manolo for their help, testing, and feeback.

fltk-options/CMakeLists.txt

@@ -1,7 +1,7 @@
 #
 # CMakeLists.txt to build fltk-options for the FLTK project using CMake (www.cmake.org)
 #
-# Copyright 2023 by Bill Spitzak and others.
+# Copyright 2023-2024 by Bill Spitzak and others.
 #
 # This library is free software. Distribution and use rights are outlined in
 # the file "COPYING" which should have been included with this file.  If this
@@ -14,71 +14,59 @@
 #     https://www.fltk.org/bugs.php
 #
 
-set (CPPFILES
-  fltk-options.cxx
-)
+# Targets that will be built: fltk-options and fltk-options-cmd (Windows)
+set(TARGETS fltk-options)
 
-set (HEADERFILES
-)
-
-set (FLTK_OPTIONS_TARGETS fltk-options)  # fltk-options and optional fltk-options-cmd target
-set (FLTK_OPTIONS_LIBS fltk)  # libraries used to link fltk-options executables
-
-if (APPLE AND (NOT OPTION_APPLE_X11))
+if(APPLE AND NOT FLTK_BACKEND_X11)
 
   # macOS
 
-  set (ICON_NAME fltk-options.icns)
-  set (ICON_PATH "${CMAKE_CURRENT_SOURCE_DIR}/icons/${ICON_NAME}")
-  add_executable (fltk-options MACOSX_BUNDLE ${CPPFILES} ${HEADERFILES} ${ICON_PATH})
+  set(ICON_NAME fltk-options.icns)
+  set(ICON_PATH "${CMAKE_CURRENT_SOURCE_DIR}/icons/${ICON_NAME}")
+  add_executable(fltk-options MACOSX_BUNDLE fltk-options.cxx ${ICON_PATH})
 
   # create macOS bundle wrapper script
 
-  set (WRAPPER "${EXECUTABLE_OUTPUT_PATH}/${CMAKE_CFG_INTDIR}/fltk-options")
-  add_custom_command (
+  set(WRAPPER "${EXECUTABLE_OUTPUT_PATH}/${CMAKE_CFG_INTDIR}/fltk-options")
+  add_custom_command(
     TARGET fltk-options POST_BUILD
     COMMAND cp ${CMAKE_CURRENT_SOURCE_DIR}/../CMake/macOS-bundle-wrapper.in ${WRAPPER}
     COMMAND chmod u+x,g+x,o+x ${WRAPPER}
     BYPRODUCTS ${WRAPPER}
     VERBATIM
   )
-  unset (WRAPPER)
+  unset(WRAPPER)
 
-else ()
+else()
 
-  # option WIN32 builds a Windows GUI program, ignored on other platforms
-  add_executable (fltk-options WIN32 ${CPPFILES} ${HEADERFILES})
+  # Option WIN32 builds a Windows GUI program, ignored on other platforms
+  add_executable(fltk-options WIN32 fltk-options.cxx)
 
-endif ()
+endif()
 
-# we must link fltk-optons with cairo if option CAIRO is enabled
-if (FLTK_HAVE_CAIRO)
-  target_link_directories (fltk-options PRIVATE ${PKG_CAIRO_LIBRARY_DIRS})
-endif (FLTK_HAVE_CAIRO)
+target_link_libraries(fltk-options PRIVATE fltk::fltk)
 
-if (USE_GDIPLUS)        # can only be true on Windows
-  list (APPEND FLTK_OPTIONS_LIBS gdiplus)
-endif (USE_GDIPLUS)
-
-target_link_libraries (fltk-options ${FLTK_OPTIONS_LIBS})
-
-
-# Add fltk-options-cmd console app (Windows only) for editing FLTK options.
+# Add the console app (Windows only)
 # This is done for all Windows targets, even if cross-compiling.
 
-if (WIN32)
-  list (APPEND FLTK_OPTIONS_TARGETS fltk-options-cmd)
-  add_executable (fltk-options-cmd ${CPPFILES} ${HEADERFILES})
-  target_link_libraries (fltk-options-cmd ${FLTK_OPTIONS_LIBS})
-  if (FLTK_HAVE_CAIRO)
-    target_link_directories (fltk-options-cmd PRIVATE ${PKG_CAIRO_LIBRARY_DIRS})
-  endif (FLTK_HAVE_CAIRO)
-endif ()
+if(WIN32)
+  list(APPEND TARGETS fltk-options-cmd)
+  add_executable(fltk-options-cmd fltk-options.cxx)
+  target_link_libraries(fltk-options-cmd PRIVATE fltk::fltk)
+endif()
 
+# Create aliases for all executables,
+# replacing 'fltk-options' with 'fltk::options'
 
-# install fltk-options GUI and commandline tool
+foreach(tgt ${TARGETS})
+  string(REPLACE "fltk-options" "fltk::options" alias ${tgt})
+  add_executable(${alias} ALIAS ${tgt})
+  unset(alias)
+endforeach()
 
-if (APPLE AND (NOT OPTION_APPLE_X11))
+# Install fltk-options GUI and commandline tool
+
+if(APPLE AND NOT FLTK_BACKEND_X11)
 
   # On macOS, fltk-options must be installed twice. The bundled version of
   # fltk-options needs to go into the /Applications folder to make it visible
@@ -90,49 +78,49 @@ if (APPLE AND (NOT OPTION_APPLE_X11))
   # one included in the bundle.
 
   # create bundle
-  set_target_properties (fltk-options PROPERTIES MACOSX_BUNDLE_INFO_PLIST "${CMAKE_CURRENT_SOURCE_DIR}/fltk-options.plist")
-  set_target_properties (fltk-options PROPERTIES MACOSX_BUNDLE_ICON_FILE ${ICON_NAME})
-  set_target_properties (fltk-options PROPERTIES XCODE_ATTRIBUTE_PRODUCT_BUNDLE_IDENTIFIER "org.fltk.fltk-options")
+  set_target_properties(fltk-options PROPERTIES MACOSX_BUNDLE_INFO_PLIST "${CMAKE_CURRENT_SOURCE_DIR}/fltk-options.plist")
+  set_target_properties(fltk-options PROPERTIES MACOSX_BUNDLE_ICON_FILE ${ICON_NAME})
+  set_target_properties(fltk-options PROPERTIES XCODE_ATTRIBUTE_PRODUCT_BUNDLE_IDENTIFIER "org.fltk.fltk-options")
 
   # The line below would wrongly install /Applications/fltk-options.icns
-  # ## set_target_properties (fltk-options PROPERTIES RESOURCE ${ICON_PATH})
+  # ## set_target_properties(fltk-options PROPERTIES RESOURCE ${ICON_PATH})
 
   # install fltk-options GUI and commandline tools
-  #install (TARGETS fltk-options DESTINATION "/Applications")
+  # install(TARGETS fltk-options DESTINATION "/Applications")
 
   # install command line tool
-  install (PROGRAMS $<TARGET_FILE:fltk-options> DESTINATION ${FLTK_BINDIR})
+  install(PROGRAMS $<TARGET_FILE:fltk-options> DESTINATION ${FLTK_BINDIR})
 
 else()
 
   # install fltk-options GUI and optional commandline tool 'fltk-options-cmd'
   # (only on Windows)
 
-  install (TARGETS ${FLTK_OPTIONS_TARGETS}
+  install(TARGETS ${TARGETS}
     EXPORT FLTK-Targets
     RUNTIME DESTINATION ${FLTK_BINDIR}
     LIBRARY DESTINATION ${FLTK_LIBDIR}
     ARCHIVE DESTINATION ${FLTK_LIBDIR}
   )
 
-  endif (APPLE AND (NOT OPTION_APPLE_X11))
+endif(APPLE AND NOT FLTK_BACKEND_X11)
 
-# install desktop files
+# Install desktop files
 
-if (UNIX)
-  install (FILES fltk-options.desktop
+if(UNIX)
+  install(FILES fltk-options.desktop
     DESTINATION ${FLTK_DATADIR}/applications
   )
-  # Install mime-type file (x-fltk-options.desktop method is deprecated)
-  install (FILES fltk-options.xml
+  # Install mime-type file(x-fltk-options.desktop method is deprecated)
+  install(FILES fltk-options.xml
     DESTINATION ${FLTK_DATADIR}/mime/packages
   )
 
   # Install desktop icons
-  foreach (icon 32 48 64 128)
-    install (FILES icons/fltk-options-${icon}.png
+  foreach(icon 32 48 64 128)
+    install(FILES icons/fltk-options-${icon}.png
       DESTINATION ${FLTK_DATADIR}/icons/hicolor/${icon}x${icon}/apps
       RENAME fltk-options.png
     )
   endforeach()
-endif (UNIX)
+endif(UNIX)