Fix build warnings

[fornellas]

This PR fixes this build warning:

# github.com/go-gl/glfw/v3.3/glfw
In file included from ../../.cache/cgs/GOMODCACHE/github.com/go-gl/glfw/v3.3/[email protected]/c_glfw_lin.go:10:
../../.cache/cgs/GOMODCACHE/github.com/go-gl/glfw/v3.3/[email protected]/glfw/src/wl_window.c:29:9: warning: ‘_GNU_SOURCE’ redefined
   29 | #define _GNU_SOURCE
      |         ^~~~~~~~~~~
<command-line>: note: this is the location of the previous definition

[Jacalz]

Thanks. As I commented on in #359 (comment), these files are imported from the upstream Glfw project. I'd suggest opening a PR upstream and making sure that your changes here are applied with a patch file one regenerating Glfw sources (see the scripts folder).