Fixed building testgles2 on Linux
diff --git a/test/testgles2.c b/test/testgles2.c
index 4a22375..65deaa5 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