Define mingw_build_win32/win64 in system macros
So far each individual spec file has had to define mingw_build_win32
and/or mingw_build_win64 on top of each spec file:
%global mingw_build_win32 1
%global mingw_build_win64 1
This commit changes it so that the default is now defined in system-wide
macros and each individual package doesn't have to clutter their spec
files with these two lines. The default is to build both 32 bit and 64
bit packages; if spec files need to opt out, they can just define either
mingw_build_win32 or mingw_build_win64 to 0.