Use LLVM Clang explicitly on FreeBSD

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)

hack/make.sh

@@ -107,6 +107,16 @@ LDFLAGS_STATIC_DOCKER="
 	-extldflags \"$EXTLDFLAGS_STATIC_DOCKER\"
 "
 
+if [ "$(uname -s)" = 'FreeBSD' ]; then
+	# Tell cgo the compiler is Clang, not GCC
+	# https://code.google.com/p/go/source/browse/src/cmd/cgo/gcc.go?spec=svne77e74371f2340ee08622ce602e9f7b15f29d8d3&r=e6794866ebeba2bf8818b9261b54e2eef1c9e588#752
+	export CC=clang
+
+	# "-extld clang" is a workaround for
+	# https://code.google.com/p/go/issues/detail?id=6845
+	LDFLAGS="$LDFLAGS -extld clang"
+fi
+
 HAVE_GO_TEST_COVER=
 if \
 	go help testflag | grep -- -cover > /dev/null \