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/pedrib/geany/commit/469db1505508e1f4b9091e2ebe9be5d7434ac...)). 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