Fix C++ on Darwin

Port of https://android-review.googlesource.com/207241

Change-Id: Idad81a528a2104579de0225b0724bb81293eea44
diff --git a/cc/toolchain.go b/cc/toolchain.go
index 9f6ad2e..1f51384 100644
--- a/cc/toolchain.go
+++ b/cc/toolchain.go
@@ -70,6 +70,9 @@
 
 	ShlibSuffix() string
 	ExecutableSuffix() string
+
+	SystemCppCppflags() string
+	SystemCppLdflags() string
 }
 
 type toolchainBase struct {
@@ -117,6 +120,14 @@
 	return ""
 }
 
+func (toolchainBase) SystemCppCppflags() string {
+	return ""
+}
+
+func (toolchainBase) SystemCppLdflags() string {
+	return ""
+}
+
 type toolchain64Bit struct {
 	toolchainBase
 }