Use a more robust way to substitute gcc for clang, when compiling

gnu/lib/libobjc and sys/boot/i386/boot2, so it also works when using
absolute paths and/or options, as in CC="/absolute/path/clang -foo".

Approved by:	rpaulo (mentor)

gnu/lib/libobjc/Makefile

@@ -16,7 +16,9 @@ SRCS=	archive.c class.c encoding.c gc.c hash.c init.c linking.m misc.c \
 	selector.c sendmsg.c thr.c thr-objc.c exception.c
 
 # XXX: clang cannot compile libobjc yet
-CC:=${CC:C/^cc|^clang/gcc/}
+.if ${CC:T:Mclang} == "clang"
+CC=gcc
+.endif
 
 INCS=	encoding.h hash.h objc-api.h objc-decls.h objc-list.h objc.h runtime.h \
 	sarray.h thr.h typedstream.h NXConstStr.h Object.h Protocol.h

sys/boot/i386/boot2/Makefile

@@ -3,7 +3,9 @@
 .include <bsd.own.mk>
 
 # XXX: clang can compile the boot code just fine, but boot2 gets too big
-CC:=${CC:C/^cc|^clang/gcc/}
+.if ${CC:T:Mclang} == "clang"
+CC=gcc
+.endif
 
 FILES=		boot boot1 boot2