ref_gl: de-duplicate GL function definitions

- Used APIENTRY_LINKAGE in ref_gl to remove duplicate definitions of GL
  functions among translation units
- Fixes building ref_gl with -fno-common (GCC 10)

ref_gl/gl_context.c

@@ -13,6 +13,8 @@ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU General Public License for more details.
 */
 
+// GL API function pointers, if any, reside in this translation unit
+#define APIENTRY_LINKAGE
 #include "gl_local.h"
 #include "gl_export.h"
 

ref_gl/gl_export.h

@@ -20,7 +20,7 @@ GNU General Public License for more details.
 #endif
 
 #ifndef APIENTRY_LINKAGE
-#define APIENTRY_LINKAGE
+#define APIENTRY_LINKAGE extern
 #endif
 
 #if defined XASH_NANOGL || defined XASH_WES || defined XASH_REGAL