ftgl: fix for LinuxBrew
ftgl: fix for LinuxBrew

Fixes:
configure: error: GL/gl.h or OpenGL/gl.h is needed, please specify its location with --with-gl-inc.

Closes Linuxbrew/homebrew-core#1814.

Signed-off-by: Maxim Belkin <032baa6ccf1d4eaa4536fc4f9b83b9a1f593449a@gmail.com>

Linuxbrew/homebrew-core,Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core,maxim-belkin/homebrew-core