gcc issued lots of warnings if compiled with -Wundef. This shows access
to undefined preprocessor macros, particularly for config tests.
This update is similar, but not identical to fltk-cpp-fixes.patch as
posted on STR #2988 (file #1). There have been changes in the configure
script and the macros in configh.in and configh.cmake.in meanwhile.
This commit compiles with -Wundef w/o such warnings under Linux and
Windows (MinGW).
git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@11094 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
