Here's a tip on how to find the correct path for setting the include directory.

When working on large C/C++ projects it can be difficult to tell where a given header is coming from, it's hard to search for this as answers found on search engines are usually specific to some project.

So how can you find where C/C++ headers are? Or which package should you include to get a specific header?

Simple, just use:

locate glx.h

Unlike find it doesn't search in realtime but it uses a database (from updatedb) so it's much faster.