cmake: append to CMAKE_MODULE_PATH instead of overwriting

CMakeLists.txt

@@ -68,7 +68,8 @@ include(CMakeDependentOption)
 include(CMakePushCheckState)
 include(FindPkgConfig)
 include(GNUInstallDirs)
-set(CMAKE_MODULE_PATH "${SDL2_SOURCE_DIR}/cmake")
+
+list(APPEND CMAKE_MODULE_PATH "${SDL2_SOURCE_DIR}/cmake")
 include(${SDL2_SOURCE_DIR}/cmake/macros.cmake)
 include(${SDL2_SOURCE_DIR}/cmake/sdlchecks.cmake)