sdl12-compat: Revert "Don't include glu.h by default"

From bef8f7412dd44edc4f7e14dc35d3b20399e25496 Mon Sep 17 00:00:00 2001
From: Sam Lantinga <[EMAIL REDACTED]>
Date: Thu, 6 Nov 2025 07:19:20 -0800
Subject: [PATCH] Revert "Don't include glu.h by default"

This reverts commit 6595f6e405844cbca4a55e47eaa29ccec0cdd75f.

It turns out there are a number of games that depend on this:
* antigrav
* armagetronad
* bloboats
* blockout2
* brutalchess
* cytadela
* enemylines3
* enemylines7
* opencity
* pathogen

Fixes https://github.com/libsdl-org/sdl12-compat/issues/382
---
 include/SDL/SDL_opengl.h | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/include/SDL/SDL_opengl.h b/include/SDL/SDL_opengl.h
index ba92b114f..776541a20 100644
--- a/include/SDL/SDL_opengl.h
+++ b/include/SDL/SDL_opengl.h
@@ -45,15 +45,11 @@ real SDL-1.2 available to you. */
 #endif
 #if defined(__MACOSX__)
 #include <OpenGL/gl.h>  /* Header File For The OpenGL Library */
-#ifdef SDL_INCLUDE_GLU_H
 #include <OpenGL/glu.h> /* Header File For The GLU Library */
-#endif
 #else
 #include <GL/gl.h>      /* Header File For The OpenGL Library */
-#ifdef SDL_INCLUDE_GLU_H
 #include <GL/glu.h>     /* Header File For The GLU Library */
 #endif
-#endif
 #ifndef NO_SDL_GLEXT
 #undef __glext_h_
 #undef __gl_glext_h_