RepositoryExternal: fix confusing condition

This looks like it does XOR but vmiklos says it should do OR.

Change-Id: I11cee255fb243d0cd763e97d13da1e31a8e598e9

RepositoryExternal.mk

@@ -183,7 +183,7 @@ endef
 
 endif # SYSTEM_CPPUNIT
 
-ifeq ($(ENABLE_OPENGL)$(if $(filter ANDROID,$(OS)),TRUE),TRUE)
+ifneq ($(ENABLE_OPENGL)$(if $(filter ANDROID,$(OS)),TRUE),)
 ifneq ($(SYSTEM_GLEW),)
 
 define gb_LinkTarget__use_glew