sdl12-compat: testgl.c has to link against OpenGL32.lib on Windows.

SDL Commits
1 
From 6a48903e0860c8b799bdfbb93cf99f1f50d008c8 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Tue, 1 Jun 2021 21:53:16 -0400
Subject: [PATCH] testgl.c has to link against OpenGL32.lib on Windows.

---
 CMakeLists.txt | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index f73ac90..cfb05c1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -139,6 +139,8 @@ if(SDL12TESTS)
     if(APPLE)
         target_link_libraries(testgl "-framework OpenGL")
         set_target_properties(testgl PROPERTIES COMPILE_DEFINITIONS "GL_SILENCE_DEPRECATION=1")
+    elseif(WIN32)
+        target_link_libraries(testgl "OpenGL32")
     else()
         target_link_libraries(testgl "GL")
     endif()