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).

1 files changed, 2 insertions, 27 deletions

diff --git a/CMake/export.cmake b/CMake/export.cmake
index 642be6762..407b24933 100644
--- a/CMake/export.cmake
+++ b/CMake/export.cmake
@@ -1,9 +1,9 @@
 #
-# Export CMake file to build the FLTK project using CMake (www.cmake.org)
+# Export CMake file to build the FLTK project using CMake
 #
 # Originally written by Michael Surette
 #
-# Copyright 1998-2024 by Bill Spitzak and others.
+# Copyright 1998-2025 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
@@ -136,31 +136,6 @@ else(CMAKE_VERSION VERSION_LESS 3.19)
                     WORLD_READ WORLD_EXECUTE)
 endif(CMAKE_VERSION VERSION_LESS 3.19)
 
-# prepare some variables for config.h
-
-if(IS_ABSOLUTE "${FLTK_DATADIR}")
-  set(PREFIX_DATA "${FLTK_DATADIR}/fltk")
-else(IS_ABSOLUTE "${FLTK_DATADIR}")
-  set(PREFIX_DATA "${CMAKE_INSTALL_PREFIX}/${FLTK_DATADIR}/fltk")
-endif(IS_ABSOLUTE "${FLTK_DATADIR}")
-
-if(IS_ABSOLUTE "${FLTK_DOCDIR}")
-  set(PREFIX_DOC "${FLTK_DOCDIR}/fltk")
-else(IS_ABSOLUTE "${FLTK_DOCDIR}")
-  set(PREFIX_DOC "${CMAKE_INSTALL_PREFIX}/${FLTK_DOCDIR}/fltk")
-endif(IS_ABSOLUTE "${FLTK_DOCDIR}")
-
-set(CONFIG_H_IN config.h.in)
-set(CONFIG_H config.h)
-
-# generate config.h
-
-configure_file(
-  "${CMAKE_CURRENT_SOURCE_DIR}/${CONFIG_H_IN}"
-  "${CMAKE_CURRENT_BINARY_DIR}/${CONFIG_H}"
-  @ONLY
-)
-
 if(FLTK_INSTALL_LINKS)
   # Set PREFIX_INCLUDE to the proper value.
   if(IS_ABSOLUTE ${FLTK_INCLUDEDIR})