When using clang, override CXX to 'c++' if CC is 'cc' and 'clang++' if 'cc'

is 'clang'

src/build/Common.gmake

@@ -158,6 +158,15 @@ endif
 CFLAGS	+=	-MMD
 CC		?=	gcc
 CCPRE	?= ${shell if [ `echo __clang__ | $(CC) -E - | grep -v '^\#'` != __clang__ ] ; then echo clang ; elif [ `echo __INTEL_COMPILER | $(CC) -E - | grep -v '^\#'` != __INTEL_COMPILER ] ; then echo icc ; else echo gcc ; fi}
+ifeq ($(CCPRE),clang)
+ ifeq ($(CC),cc)
+  CXX	:=	c++
+ else
+  ifeq ($(CC),clang)
+   CXX	:=	clang++
+  endif
+ endif
+endif
 ifdef BUILD_DEPENDS
  CC				:=	$(SRC_ROOT)/build/mkdep -a
  CXX			:=	$(SRC_ROOT)/build/mkdep -a