diff --git a/share/mk/bsd.sys.mk b/share/mk/bsd.sys.mk
--- a/share/mk/bsd.sys.mk
+++ b/share/mk/bsd.sys.mk
@@ -273,7 +273,14 @@
 # Only allow .TARGET when not using PROGS as it has the same syntax
 # per PROG which is ambiguous with this syntax. This is only needed
 # for PROG_VARS vars.
-.if !defined(_RECURSING_PROGS)
+#
+# Some directories (currently just clang) also need to disable this since
+# CFLAGS.${COMPILER_TYPE}, CFLAGS.${.IMPSRC:T} and CFLAGS.${.TARGET:T} all live
+# in the same namespace, meaning that, for example, GCC builds of clang pick up
+# CFLAGS.clang via CFLAGS.${.TARGET:T} and thus try to pass Clang-specific
+# flags. Ideally the different sources of CFLAGS would be namespaced to avoid
+# collisions.
+.if !defined(_RECURSING_PROGS) && !defined(NO_TARGET_FLAGS)
 .if ${MK_WARNS} != "no"
 CFLAGS+=	${CWARNFLAGS.${.TARGET:T}}
 .endif
diff --git a/usr.bin/clang/clang/Makefile b/usr.bin/clang/clang/Makefile
--- a/usr.bin/clang/clang/Makefile
+++ b/usr.bin/clang/clang/Makefile
@@ -34,4 +34,7 @@
 
 LIBADD+=	z
 
+# Ensure we don't add CFLAGS.clang when using GCC
+NO_TARGET_FLAGS=
+
 .include "../clang.prog.mk"