Error out if <prex.h> is included when compiling for OS X

The X11 headers are present in the OS X SDK (because OS X does have an X11
server, even though I think it might be a separate download and install on
demand nowadays), but we don't want to include them even if it would be
harmless.

Change-Id: I48011d7e1d2cd6398fc7b4cba8ec8100addc053b

include/prex.h

@@ -20,6 +20,10 @@
 #ifndef _PREX_H
 #define _PREX_H
 
+#ifdef MACOSX
+#error No X headers should be included for MACOSX even if they are present in the SDK
+#endif
+
 /* Types from <X11/X.h> that clash with LO's identifiers
  * and we don't need.
  */