tightly couple protoc-c output with the protobuf-c headers

this adds a version guard like the protobuf C++ implementation. it
ensures that protoc-c and <protobuf-c.h> are from the exact same version
of protobuf-c.

protoc-c/c_file.cc

@@ -32,7 +32,8 @@
 //  Based on original Protocol Buffers design by
 //  Sanjay Ghemawat, Jeff Dean, and others.
 
-// Copyright (c) 2008-2013, Dave Benson.  All rights reserved.
+// Copyright (c) 2008-2014, Dave Benson and the protobuf-c authors.
+// All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
@@ -69,6 +70,8 @@
 #include <google/protobuf/io/printer.h>
 #include <google/protobuf/descriptor.pb.h>
 
+#include "protobuf-c.h"
+
 namespace google {
 namespace protobuf {
 namespace compiler {
@@ -131,25 +134,21 @@ void FileGenerator::GenerateHeader(io::Printer* printer) {
     "filename", file_->name(),
     "filename_identifier", filename_identifier);
 
-#if 0
-  // Verify the protobuf library header version is compatible with the protoc
-  // version before going any further.
+  // Verify the protobuf-c library header version is compatible with the
+  // protoc-c version before going any further.
   printer->Print(
-    "#if GOOGLE_PROTOBUF_VERSION < $min_header_version$\n"
-    "#error This file was generated by a newer version of protoc which is\n"
-    "#error incompatible with your Protocol Buffer headers.  Please update\n"
-    "#error your headers.\n"
-    "#endif\n"
-    "#if $protoc_version$ < GOOGLE_PROTOBUF_MIN_PROTOC_VERSION\n"
-    "#error This file was generated by an older version of protoc which is\n"
-    "#error incompatible with your Protocol Buffer headers.  Please\n"
-    "#error regenerate this file with a newer version of protoc.\n"
+    "#if PROTOBUF_C_VERSION_NUMBER < $min_header_version$\n"
+    "# error This file was generated by a newer version of protoc-c which is "
+    "incompatible with your libprotobuf-c headers. Please update your headers.\n"
+    "#elif $protoc_version$ < PROTOBUF_C_VERSION_NUMBER\n"
+    "# error This file was generated by an older version of protoc-c which is "
+    "incompatible with your libprotobuf-c headers. Please regenerate this file "
+    "with a newer version of protoc-c.\n"
     "#endif\n"
     "\n",
     "min_header_version",
-      SimpleItoa(protobuf::internal::kMinHeaderVersionForProtoc),
-    "protoc_version", SimpleItoa(GOOGLE_PROTOBUF_VERSION));
-#endif
+      SimpleItoa(PROTOBUF_C_VERSION_NUMBER),
+    "protoc_version", SimpleItoa(PROTOBUF_C_VERSION_NUMBER));
 
   for (int i = 0; i < file_->dependency_count(); i++) {
     printer->Print(