Improve procedure to construct best link command for macOS platform

As discussed between Matthias, Albrecht and Manolo, the best procedure is
- use MAC_OS_X_VERSION_MAX_ALLOWED but not __MAC_OS_X_VERSION_MAX_ALLOWED;
- compile helper file CMake /macOSMaxAllowed.c to detect whether the SDK in use
is in version above a given threshold version number;
- based upon the result of this check, decide to weakly link or not a given framework.

CMake/macOSMaxAllowed.c

@@ -1,6 +1,6 @@
 
 #include <AvailabilityMacros.h>
-#if __MAC_OS_X_VERSION_MAX_ALLOWED < SDK_VERSION_CHECK
-#error __MAC_OS_X_VERSION_MAX_ALLOWED < SDK_VERSION_CHECK
+#if MAC_OS_X_VERSION_MAX_ALLOWED < SDK_VERSION_CHECK
+#error MAC_OS_X_VERSION_MAX_ALLOWED < SDK_VERSION_CHECK
 #endif
 int main(int argc, char** argv) { return 0; }