Remove hardcoded version numbers: part 1

The goal is to change the version number for a new release only in
CMakeLists.txt. This is the first step.

Details:

- CMake/gen_config.cmake: this new file is included to generate the
  header files config.h (private, root directory), and FL/fl_config.h
  (public, can be installed). This file implements also ABI version
  checks (removed from FL/Enumerations.H and with more checks).
  Warnings are issued if the chosen ABI version is invalid.
- CMake/export.cmake: code to generate 'config.h' was moved to
  CMake/gen_config.cmake.
- CMake/options.cmake: set default of FLTK_BUILD_FORMS=OFF + comments
- CMakeLists.txt: move generation of FL/fl_config.h to gen_config.cmake,
  add API and ABI versions to CMake summary,
- FL/Enumerations.H: remove most of the version number details which
  are now included in FL/fl_config.h. This needed also some doxygen
  related changes.
- README.CMake.txt: improve docs of FL_ABI_VERSION and some more.
  Reflect the new default of CMake option FLTK_BUILD_FORMS (OFF).
- documentation/Doxyfile.in: add FL/fl_config.h to file list. This
  file is created in the build tree (and may be "installed").
- fl_config.h.in: add version number details that have been moved here
  from Enumerations.H (used to generate FL/fl_config.h).

documentation/Doxyfile.in

@@ -797,6 +797,7 @@ INPUT                  = @CMAKE_CURRENT_SOURCE_DIR@/src/index.dox \
                          @CMAKE_CURRENT_SOURCE_DIR@/src/fltk-options.dox \
                          @CMAKE_CURRENT_SOURCE_DIR@/src/advanced.dox \
                          @CMAKE_CURRENT_SOURCE_DIR@/src/unicode.dox \
+                         @CMAKE_CURRENT_BINARY_DIR@/../FL/fl_config.h \
                          @FLTK_SOURCE_DIR@/FL \
                          @FLTK_SOURCE_DIR@/src \
                          @CMAKE_CURRENT_SOURCE_DIR@/src/enumerations.dox \