Originally Posted by manishsingh4u
And, depending on how your system is set up: /usr/local/include /usr/X11R6/include. Also, what you're looking for might be in an installation-specific directory, e.g. /usr/local/Mesa/include.
As far as I know, header files typically don't have version numbers, so you only have to worry about big things like the difference between, for example, GTK and GTK2, which each have their own headers.