Improve doxygen documentation generation

Under configure+make it is not possible to enable driver documentation.
Therefore the '@DRIVER_DOCS@' variable must always be cleared.
Add a comment on how to build the driver docs using the Makefile.

Simplify CMake whitespace (trailing LF) handling.

Make sure we find the correct source dir even if FLTK is included
in a sub build (s/CMAKE_SOURCE_DIR/FLTK_SOURCE_DIR/).

documentation/CMakeLists.txt

@@ -1,7 +1,7 @@
 #
 # CMakeLists.txt to build docs 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
@@ -40,11 +40,13 @@ if (GENERATE_DOCS)
 
   execute_process (COMMAND date "+%Y"
     OUTPUT_VARIABLE YEAR
+    OUTPUT_STRIP_TRAILING_WHITESPACE
   )
 
   # note: current locale is used for abbreviated month
   execute_process (COMMAND date "+%b %d, %Y"
     OUTPUT_VARIABLE CURRENT_DATE
+    OUTPUT_STRIP_TRAILING_WHITESPACE
   )
 
   # find git revision
@@ -55,12 +57,12 @@ if (GENERATE_DOCS)
   #        that contains the git revision.
 
   execute_process (COMMAND
-    git --git-dir=${CMAKE_SOURCE_DIR}/.git rev-parse --short=10 HEAD
+    git --git-dir=${FLTK_SOURCE_DIR}/.git rev-parse --short=10 HEAD
     OUTPUT_VARIABLE GIT_REVISION
+    OUTPUT_STRIP_TRAILING_WHITESPACE
     ERROR_QUIET
   )
-  # strip trailing newline from git revision
-  string (REPLACE "\n" "" GIT_REVISION "${GIT_REVISION}")
+
   # set to "'unkown'" if git is not available
   if (GIT_REVISION STREQUAL "")
     set (GIT_REVISION "'unkown'")
@@ -69,6 +71,7 @@ if (GENERATE_DOCS)
   # find doxygen version
 
   if (DOXYGEN_FOUND)
+
     # strip trailing git revision if doxygen was built from source
     string (REGEX REPLACE " .*$" "" DOXYGEN_VERSION_SHORT ${DOXYGEN_VERSION})
     # strip trailing newline
@@ -90,6 +93,8 @@ if (GENERATE_DOCS)
   )
 
   if (0) # debug
+    fl_debug_var (YEAR)
+    fl_debug_var (CURRENT_DATE)
     fl_debug_var (GIT_REVISION)
     fl_debug_var (DOXYGEN_FOUND)
     fl_debug_var (DOXYGEN_EXECUTABLE)

documentation/Makefile

@@ -138,6 +138,15 @@ uninstall-linux uninstall-osx:
 # Both files are generated from the common source file Doxyfile.in.
 # Note that Doxyfile.in is shared with CMake to configure these files.
 
+# Note: There's no way to enable the "Driver Documentation" via configure+make,
+#       please use CMake 'OPTION_INCLUDE_DRIVER_DOCUMENTATION' instead.
+#       Alternatively (if you really need it) edit this Makefile and
+#       replace the two lines below containing "@DRIVER_DOCS@" to read:
+#	    -e's, @DRIVER_DOCS@, DriverDev,' \
+#       (w/o leading '#') to enable the "Driver Documentation" section.
+#       Please take care of copying tabs and spaces literally.
+# Note: There's no intention to enable this via configure option (use CMake).
+
 Doxyfile: Doxyfile.in generated.dox copyright.dox
 	echo "Generating Doxyfile ..."
 	$(DOXYDOC) -u -s - < $< | \
@@ -145,6 +154,7 @@ Doxyfile: Doxyfile.in generated.dox copyright.dox
 	    -e's,@GENERATE_HTML@,YES,' \
 	    -e's,@GENERATE_LATEX@,NO,' \
 	    -e's, @LATEX_HEADER@,,' \
+	    -e's, @DRIVER_DOCS@,,' \
 	    -e's,@CMAKE_CURRENT_SOURCE_DIR@,.,' \
 	    -e's,@CMAKE_CURRENT_BINARY_DIR@,,' \
 	    -e's,@FLTK_SOURCE_DIR@,..,' \
@@ -157,6 +167,7 @@ Doxybook: Doxyfile.in generated.dox copyright.dox
 	    -e's,@GENERATE_HTML@,NO,' \
 	    -e's,@GENERATE_LATEX@,YES,' \
 	    -e's,@LATEX_HEADER@,src/fltk-book.tex,' \
+	    -e's, @DRIVER_DOCS@,,' \
 	    -e's,@CMAKE_CURRENT_SOURCE_DIR@,.,' \
 	    -e's,@CMAKE_CURRENT_BINARY_DIR@,,' \
 	    -e's,@FLTK_SOURCE_DIR@,..,' \