CMake: Fix linker issues on OS X and Arch Linux (STR #3045).


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10091 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
CMake/MinGW: Fix missing -pthread linker option for MinGW build.


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10092 ea41ed52-d2ee-0310-a9c1-e6b18d33e121