Bump minimum required header version for proto3 syntax

The changes in #274 (proto3: make strings default to "" instead of NULL)
add a new symbol to the library and add an 'extern' declaration to the
protobuf-c header file.

Since the compiler may generate files that depend on that new
'protobuf_c_empty_string' declaration in protobuf-c.h, we need to bump
the min_header_version value that is written into generated header
files. But since the 'protobuf_c_empty_string' symbol can only be
referenced when generating proto3 syntax files, we only need to use the
stricter min_header_version value when processing a proto3 file.

protoc-c/c_file.cc

@@ -119,7 +119,10 @@ FileGenerator::~FileGenerator() {}
 void FileGenerator::GenerateHeader(io::Printer* printer) {
   string filename_identifier = FilenameIdentifier(file_->name());
 
-  static const int min_header_version = 1000000;
+  int min_header_version = 1000000;
+  if (file_->syntax() == FileDescriptor::SYNTAX_PROTO3) {
+    min_header_version = 1003000;
+  }
 
   // Generate top of header.
   printer->Print(