Although some Windows compilers still #define WIN32, more and more
standard compliant compilers only #define _WIN32. FLTK *did* #define
WIN32 for all Windows targets with autoconf/configure, but that bit
was lost for CMake builds. This is necessary until we replace WIN32
with _WIN32 throughout the Windows specific code.
git-svn-id: file:///fltk/svn/fltk/branches/branch-1.4@12612 ea41ed52-d2ee-0310-a9c1-e6b18d33e121