fltk: fix on darwin

Compiling Fl_cocoa.mm...
Fl_cocoa.mm:4079:30: error: use of undeclared identifier 'version'; did
you mean 'Fl::version'?
    NSOperatingSystemVersion version = [[NSProcessInfo processInfo]
operatingSystemVersion];
                             ^~~~~~~
                             Fl::version
../FL/Fl.H:262:17: note: 'Fl::version' declared here
  static double version();
                ^
Fl_cocoa.mm:4079:5: error: use of undeclared identifier
'NSOperatingSystemVersion'
    NSOperatingSystemVersion version = [[NSProcessInfo processInfo]
operatingSystemVersion];
    ^
Fl_cocoa.mm:4080:9: error: use of undeclared identifier 'version'
    M = version.majorVersion;
        ^
Fl_cocoa.mm:4081:9: error: use of undeclared identifier 'version'
    m = version.minorVersion;
        ^
Fl_cocoa.mm:4082:9: error: use of undeclared identifier 'version'
    b = version.patchVersion;
        ^
5 errors generated.