diff --git a/cc/cc.go b/cc/cc.go
index 529a329..33c9611 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -97,6 +97,7 @@
 	ConlyFlags  []string // Flags that apply to C source files
 	CppFlags    []string // Flags that apply to C++ source files
 	YaccFlags   []string // Flags that apply to Yacc source files
+	protoFlags  []string // Flags that apply to proto source files
 	LdFlags     []string // Flags that apply to linker command lines
 	libFlags    []string // Flags to add libraries early to the link order