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.

libdecor/build/fl_libdecor.c

@@ -96,12 +96,12 @@ LIBDECOR_EXPORT void libdecor_frame_set_minimized(struct libdecor_frame *frame)
     * if FLTK was built with package libgtk-3-dev, the GTK plugin is used
     * if FLTK was built without package libgtk-3-dev, the Cairo plugin is used
  
- If FLTK was built with OPTION_USE_SYSTEM_LIBDECOR turned ON, the present modification
+ If FLTK was built with FLTK_USE_SYSTEM_LIBDECOR turned ON, the present modification
  isn't compiled, so the plugin-searching algorithm of libdecor_new() in libdecor-0.so is used.
  This corresponds to step 1) above and to use no titlebar is no plugin is found.
  
  N.B.: only the system package is built with a meaningful value of -DLIBDECOR_PLUGIN_DIR=
- so a plugin may be loaded that way only if FLTK was built with OPTION_USE_SYSTEM_LIBDECOR turned ON.
+ so a plugin may be loaded that way only if FLTK was built with FLTK_USE_SYSTEM_LIBDECOR turned ON.
  
  */
 LIBDECOR_EXPORT struct libdecor *libdecor_new(struct wl_display *wl_display, struct libdecor_interface *iface)