Hi,

Installing package glfw-devel from EPEL without pre-existing OpenGL headers makes the GLFW headers unusable.

It seems that the issue is caused by the package not depending on mesa-libGL-devel which provides the `gl.h` header needed in the `glfw3.h` file.

Steps to reproduce on a fresh CentOS 7 installation:
======
$ sudo yum install -y glfw-devel gcc
[...]
$ echo "#include <GLFW/glfw3.h>" > test.c
$ gcc -c test.c
In file included from test.c:1:0:
/usr/include/GLFW/glfw3.h:171:21: fatal error: GL/gl.h: No such file or directory
   #include <GL/gl.h>
                     ^
compilation terminated.
$ sudo yum install -y mesa-libGL-devel
[...]
$ gcc -c test.c
$ echo $?
0
======

Cheers.