[CMake] Simplify CMake build files, remove redundancies.
Ports of branch-1.3, svn r 11442 and 11444:
Remove src/fl_call_main.c from non-Windows (static) libraries. It would
be an empty object file anyway.
Use CMake variables to set up used files and linked libraries to remove
redundancies. All files and libs are used only in the definition of
CMake variables and used later in static and shared builds, resp.
git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3-porting@11447 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
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