Sorry everyone, I had this sitting on my computer for six months and completely forgot about it. I ported @linvinus patch to the latest git ([469db15](https://github.com/geany/geany/commit/469db1505508e1f4b9091e2ebe9be5d7434ac9...)). I only fixed some porting issues, otherwise, it's 100% his work untouched.
It seems to work beautifully :D
However I HAVE NOT TESTED IT PROPERLY! Please test it out to see if it's suitable for integration into geany: https://github.com/pedrib/geany/tree/highlight_c_cpp
@iloveclang if you wanna help please test it thoroughly, with huge, big, small and tiny source code files, multiple files opened at a time, C, C++, etc.
Since I'm not happy with my own testing, I'll wait for at least 5 or 10 other people to test it and say it works before submitting an "official" PR.