Build host tests with Clang when CLANGBUILD is set.

Change-Id: I32e04903c242d5e08f0f3bec5430dc21a27ba07d
diff --git a/make/host_test.mk b/make/host_test.mk
index a49f86b..3d552c4 100644
--- a/make/host_test.mk
+++ b/make/host_test.mk
@@ -37,7 +37,13 @@
 $(error HOST_SRCS must be specified)
 endif
 
+ifeq ($(call TOBOOL,$(CLANGBUILD)),true)
+HOST_CC := $(CLANG_BINDIR)/clang
+else
+# TODO: use hermetic version of GCC.
+# To do this, we'd need to compile boringssl from source rather than using a system library.
 HOST_CC := gcc
+endif
 
 HOST_INCLUDE_DIRS += $(GLOBAL_UAPI_INCLUDES) $(GLOBAL_SHARED_INCLUDES) $(GLOBAL_USER_INCLUDES)