GLX implementation and *nix-specific file handling implementation

I added gl-x11 which allows compatibility with X11 (Xlib-based) and GLX.
I also added various functions to handle file finding based on FHS.
Various changes to autotools to both install files correctly and to configure correctly.

libobs/graphics/graphics.h

@@ -414,9 +414,8 @@ struct gs_window {
 	void                    *hwnd;
 #elif defined(__APPLE__)
 	__unsafe_unretained id  view;
-#elif defined(__posix__)
-	int bla;
-	/* TODO */
+#elif defined(__linux__)
+	uint32_t id;
 #endif
 };