Implement wxCheckOsVersion() with NSProccessInfo on OS X.

Since OS X 10.10 NSProcessInfo has a method isOperatingSystemAtLeastVersion: which should be used compare the OS version if necessary.

src/unix/utilsunx.cpp

@@ -1148,8 +1148,6 @@ wxString wxGetOsDescription()
     return wxGetCommandOutput(wxT("uname -s -r -m"));
 }
 
-#endif // !__DARWIN__
-
 bool wxCheckOsVersion(int majorVsn, int minorVsn)
 {
     int majorCur, minorCur;
@@ -1158,6 +1156,8 @@ bool wxCheckOsVersion(int majorVsn, int minorVsn)
     return majorCur > majorVsn || (majorCur == majorVsn && minorCur >= minorVsn);
 }
 
+#endif // !__DARWIN__
+
 unsigned long wxGetProcessId()
 {
     return (unsigned long)getpid();