C++ Generator, improve include guard generation

We integrated avro into our cmake build system. So we have json -> c++ header build step which is quite similar to the QT moc-compiler. This build step will overwrite the target header-files if the json schema file was modified or when the generated header files are different.
In addition to that we want to put the generated header files under version control.
However, the generated include-guards contains some random parts. This random part troubles the version control system. In addition to that this random part leads to unnecessary rebuilds as cmake 'thinks' important files have been changed and triggers a rebuild of the corresponding dependencies.

Suggestion:
Add an additional command line parameter to either switch off the random part of the string, or to take a string from the command line and use this as include guard.