Add a 'make doc' target in the main Makefile

It first build the RGL, then calls doc/Makefile,
with the corrent GF_LIB_PATH, to build the documentation.

Makefile

@@ -2,7 +2,7 @@
 
 RUNMAKE=runghc Make.hs
 
-.PHONY: build copy install clean
+.PHONY: build copy install doc clean
 
 default: build copy
 
@@ -14,5 +14,8 @@ copy:
 
 install: build copy
 
+doc: build
+	make -C doc GF_LIB_PATH=../dist
+
 clean:
 	$(RUNMAKE) clean