Fix glibc build & other Linux warnings

CMakeLists.txt

@@ -5,14 +5,18 @@ set(TARGET padlab)
 option(USE_OPENGL "Use legacy OpenGL for drawing" ON)
 
 set(CMAKE_C_STANDARD 99)
-if (C_COMPILER_ID IN_LIST "GNU;Clang;AppleClang")
-	set(GNU)
+set(GNU_COMPILERS GNU Clang AppleClang)
+if (CMAKE_C_COMPILER_ID IN_LIST GNU_COMPILERS)
+	set(GNU 1)
 elseif (MSVC)
 	string(REPLACE "/W3" "/W4" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
 endif()
 
 find_package(SDL2 REQUIRED)
 if (USE_OPENGL)
+	if (NOT DEFINED OpenGL_GL_PREFERENCE)
+		set(OpenGL_GL_PREFERENCE "GLVND")
+	endif()
 	find_package(OpenGL REQUIRED)
 endif()