Cocoa: Fix macro redefinition warning

This avoids generating a macro redefinition warning when the
GL_SILENCE_DEPRECATION macro has been defined for the whole of a larger
project.

Closes #1840.

src/cocoa_platform.h

@@ -31,7 +31,9 @@
 
 // NOTE: All of NSGL was deprecated in the 10.14 SDK
 //       This disables the pointless warnings for every symbol we use
+#ifndef GL_SILENCE_DEPRECATION
 #define GL_SILENCE_DEPRECATION
+#endif
 
 #if defined(__OBJC__)
 #import <Cocoa/Cocoa.h>