Add options to limit non-PHONY target prefixes

To help catch writing into the source directory, allow the build to
specify a list of valid prefixes that non-PHONY targets may have.

Change-Id: I90c9eb384557030a7635d8a2715aaba37bef87bf
diff --git a/flags.h b/flags.h
index d19a3b2..ce32ee8 100644
--- a/flags.h
+++ b/flags.h
@@ -51,6 +51,7 @@
   bool werror_real_to_phony;
   bool warn_phony_looks_real;
   bool werror_phony_looks_real;
+  bool werror_writable;
   const char* goma_dir;
   const char* ignore_dirty_pattern;
   const char* no_ignore_dirty_pattern;
@@ -64,6 +65,7 @@
   vector<const char*> subkati_args;
   vector<Symbol> targets;
   vector<StringPiece> cl_vars;
+  vector<string> writable;
 
   void Parse(int argc, char** argv);
 };