clang.mak: return weak assignment of clang

I thought make assigns default values for CC/CXX, but it appears
not always true.

Author: stsp

Diff for: fdpp/clang.mak

@@ -10,6 +10,9 @@ else
 $(error binutils-x86-64-linux-gnu not installed)
 endif
 endif
+
+CC ?= clang
+CXX ?= clang++
 CLANG_VER := $(shell $(CXX) --version 2>&1 | head -n 1 | grep clang | \
   sed -E 's/.+ version ([^.]+)\.[^.]+\.[^ ]+.*/\1/')
 ifeq ($(CLANG_VER),)