CMake: build shared libs with OPTION_CAIROEXT (issue #250)

- remove separate libfltk_cairo to avoid cyclic dependencies, but
- keep a dummy libfltk_cairo in 1.4.0 for backwards compatibility
- move cairo/Fl_Cairo.cxx to src/Fl_Cairo.cxx
- add preliminary Cairo support for Visual Studio (MSVC)

Static linking is not affected by this change, but users building
with hand-made Makefiles will have to remove libfltk_cairo starting
with FLTK 1.4.0. The dummy library can be linked for backwards
compatibility but it will be removed later (in 1.4.x or 1.5.0).

The shared FLTK library libfltk.{so|dylib|dll|..} depends on libcairo
if and only if FLTK is built with one of the Cairo options. This has
always been the case for OPTION_CAIROEXT but is now also true if only
OPTION_CAIRO is selected, i.e. FLTK programs linked with a Cairo
enabled shared FLTK library will also be linked with libcairo. The same
is true for configure options --enable-cairo and --enable-cairoext,
respectively.

Preliminary Cairo support for MSVC now detects a Cairo installation
using the CMake variable FLTK_CAIRO_DIR which must be set by the user.
Note that this feature is temporary and may be changed in the future
for a better and more comfortable version.

fluid/CMakeLists.txt

@@ -1,7 +1,7 @@
 #
 # CMakeLists.txt to build fluid for the FLTK project using CMake (www.cmake.org)
 #
-# Copyright 1998-2022 by Bill Spitzak and others.
+# Copyright 1998-2023 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
@@ -114,13 +114,12 @@ else ()
 
 endif ()
 
-# we must link all programs with fltk_cairo if option CAIROEXT is enabled
-if (FLTK_HAVE_CAIROEXT)
-  list (APPEND FLUID_LIBS fltk_cairo cairo)
-endif (FLTK_HAVE_CAIROEXT)
-
+# we must link fluid with Cairo if OPTION_CAIRO is enabled
 if (FLTK_HAVE_CAIRO)
-  fl_target_link_directories (fluid PRIVATE "${PKG_CAIRO_LIBRARY_DIRS}")
+  target_include_directories (fluid PRIVATE ${PKG_CAIRO_INCLUDE_DIRS})
+  if (PKG_CAIRO_LIBRARY_DIRS)
+    fl_target_link_directories (fluid PRIVATE ${PKG_CAIRO_LIBRARY_DIRS})
+  endif ()
 endif (FLTK_HAVE_CAIRO)
 
 if (USE_GDIPLUS)        # can only be true on Windows
@@ -137,11 +136,27 @@ if (WIN32)
   list (APPEND FLUID_TARGETS fluid-cmd)
   add_executable (fluid-cmd ${CPPFILES} ${HEADERFILES})
   target_link_libraries (fluid-cmd ${FLUID_LIBS})
-  if (FLTK_HAVE_CAIRO)
-    fl_target_link_directories (fluid-cmd PRIVATE "${PKG_CAIRO_LIBRARY_DIRS}")
-  endif (FLTK_HAVE_CAIRO)
-endif ()
 
+  # we must link fluid-cmd with Cairo if OPTION_CAIRO is enabled (same as above)
+  if (FLTK_HAVE_CAIRO)
+    target_include_directories (fluid-cmd PRIVATE ${PKG_CAIRO_INCLUDE_DIRS})
+    if (PKG_CAIRO_LIBRARY_DIRS)
+      fl_target_link_directories (fluid-cmd PRIVATE ${PKG_CAIRO_LIBRARY_DIRS})
+    endif ()
+  endif (FLTK_HAVE_CAIRO)
+endif (WIN32)
+
+# Build fluid-shared (experimental)
+
+if (OPTION_BUILD_SHARED_LIBS)
+  list (APPEND FLUID_TARGETS fluid-shared)
+  add_executable (fluid-shared ${CPPFILES} ${HEADERFILES})
+  if (MSVC)
+    target_link_libraries (fluid-shared fltk_SHARED)
+  else ()
+    target_link_libraries (fluid-shared fltk_images_SHARED)
+  endif (MSVC)
+endif ()
 
 # install fluid GUI and commandline tool
 
@@ -164,7 +179,7 @@ if (APPLE AND (NOT OPTION_APPLE_X11))
   # ## set_target_properties (fluid PROPERTIES RESOURCE ${ICON_PATH})
 
   # install fluid GUI and commandline tools
-  #install (TARGETS fluid DESTINATION "/Applications")
+  # install (TARGETS fluid DESTINATION "/Applications")
 
   # install command line tool
   install (PROGRAMS $<TARGET_FILE:fluid> DESTINATION ${FLTK_BINDIR})
@@ -180,7 +195,7 @@ else()
     ARCHIVE DESTINATION ${FLTK_LIBDIR}
   )
 
-  endif (APPLE AND (NOT OPTION_APPLE_X11))
+endif (APPLE AND (NOT OPTION_APPLE_X11))
 
 # install desktop files