Recommended in https://github.com/scottdraves/electricsheep/issues/72#issuecomment-455900619
--- Client/Makefile.am.orig	2019-01-20 20:50:29 UTC
+++ Client/Makefile.am
@@ -12,6 +12,7 @@ endif
 AM_CPPFLAGS = \
 	-DPACKAGE_DATA_DIR=\""$(datadir)"\" \
 	-DPACKAGE_LOCALE_DIR=\""$(datadir)/locale"\" \
+	-DGL_GLEXT_PROTOTYPES \
 	-I $(top_srcdir) \
 	-iquote ../DisplayOutput/OpenGL \
 	-I ../Common \