sdl12-compat: cmake: Fix OpenGL linking

From 1eb90cbab9d1f551be642933f1f5d23e7b5cabba Mon Sep 17 00:00:00 2001
From: "Haelwenn (lanodan) Monnier" <[EMAIL REDACTED]>
Date: Fri, 4 Jun 2021 00:37:50 +0200
Subject: [PATCH] cmake: Fix OpenGL linking

This fixes building on Linux with libOpenGL (GLVND-specific) instead of libGL (legacy, GLX-specific)
---
 CMakeLists.txt | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2231d1a..76f3b3e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -141,13 +141,20 @@ if(SDL12TESTS)
     test_program(threadwin "test/threadwin.c")
     test_program(torturethread "test/torturethread.c")
 
+    if(WIN32)
+      target_link_libraries(testgl "OpenGL32")
+    else()
+      find_package(OpenGL COMPONENTS OpenGL REQUIRED)
+
+      if(CMAKE_VERSION VERSION_LESS 3.10)
+        target_link_libraries(testgl ${OPENGL_gl_LIBRARY})
+      else()
+        target_link_libraries(testgl ${OPENGL_opengl_LIBRARY})
+      endif()
+    endif()
+
     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()
 
     foreach(fname "icon.bmp" "moose.dat" "picture.xbm" "sail.bmp" "sample.bmp" "sample.wav" "utf8.txt")