SDL: Fixed building testgles2 on Linux

From e5d59fa77ce4cd4919e4745fd0abf408409f794e Mon Sep 17 00:00:00 2001
From: Sam Lantinga <[EMAIL REDACTED]>
Date: Wed, 22 Sep 2021 09:31:26 -0700
Subject: [PATCH] Fixed building testgles2 on Linux

---
 test/testgles2.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/test/testgles2.c b/test/testgles2.c
index 4a223755aa..65deaa5666 100644
--- a/test/testgles2.c
+++ b/test/testgles2.c
@@ -22,8 +22,10 @@
 
 #if defined(__IPHONEOS__) || defined(__ANDROID__) || defined(__EMSCRIPTEN__) || defined(__NACL__) \
     || defined(__WINDOWS__) || defined(__LINUX__)
+#ifndef HAVE_OPENGLES2
 #define HAVE_OPENGLES2
 #endif
+#endif
 
 #ifdef HAVE_OPENGLES2