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.