don't build FTGL if GL is not build. This was a regression recently introduced.

git-svn-id: http://root.cern.ch/svn/root/trunk@20855 27541ba8-7e3a-0410-8455-c3a389f83636

Makefile

@@ -73,10 +73,10 @@ SYSTEMO       = $(UNIXO)
 SYSTEMDO      = $(UNIXDO)
 endif
 endif
+ifeq ($(BUILDGL),yes)
 ifeq ($(BUILDFTGL),yes)
 MODULES      += ftgl
 endif
-ifeq ($(BUILDGL),yes)
 MODULES      += gl
 endif
 ifeq ($(BUILDMYSQL),yes)