Change build infrastructure to allow building both gcc and clang. This

doesn't actually flip the switch yet, so aarch64 continues to be the only
architecture for which we build clang.
ok jsg@, deraadt@

src/include/Makefile

@@ -1,4 +1,4 @@
-#	$OpenBSD: Makefile,v 1.218 2017/03/12 23:28:13 guenther Exp $
+#	$OpenBSD: Makefile,v 1.219 2017/04/17 15:53:21 kettenis Exp $
 #	$NetBSD: Makefile,v 1.59 1996/05/15 21:36:43 jtc Exp $
 
 #	@(#)Makefile	5.45.1.1 (Berkeley) 5/6/91
@@ -42,13 +42,15 @@ RDIRS=	../lib/libcurses ../lib/libedit \
 	../usr.bin/lex ../gnu/lib/libreadline \
 	../sys/arch/${MACHINE}
 
-.if ${COMPILER_VERSION:L} == "gcc3"
+.if ${BUILD_GCC3:L} == "yes"
 RDIRS+= ../gnu/usr.bin/gcc ../gnu/lib/libobjc
 PRDIRS+= ../gnu/lib/libstdc++
-.elif ${COMPILER_VERSION:L} == "gcc4"
+.endif
+.if ${BUILD_GCC4:L} == "yes"
 RDIRS+= ../gnu/usr.bin/cc/libobjc
 PRDIRS+= ../gnu/lib/libstdc++-v3 ../gnu/usr.bin/cc/include
-.elif ${COMPILER_VERSION:L} == "clang"
+.endif
+.if ${BUILD_CLANG:L} == "yes"
 RDIRS+= ../lib/libcxxabi ../lib/libcxx
 .endif