make: override built-in CXX var [#182]

make sets CXX to gcc.
We can clear it by the simple assignment:
CXX=
That assignment is ignored if the var was set from cmd line.
Using -R seems to be no longer needed.

fdpp/clang.mak

@@ -3,6 +3,9 @@
 #
 
 CC = clang++
+# Override builtin CXX.
+# The assignment below is ignored if CXX was set via cmd line.
+CXX=
 ifeq ($(CXX),)
 CL = clang++
 else

makefile

@@ -6,7 +6,7 @@ define mdir
 	    mkdir $(1) 2>/dev/null ; \
 	    ln -s $(srcdir)/$(1)/makefile $(1)/makefile ; \
 	fi
-	cd $(1) && $(MAKE) -R srcdir=$(srcdir)/$(1) $(2)
+	cd $(1) && $(MAKE) srcdir=$(srcdir)/$(1) $(2)
 endef
 
 all: