diff -Naur external_openal_original/CMakeLists.txt external_openal/CMakeLists.txt
--- external_openal_original/CMakeLists.txt	2016-01-24 20:12:39 -0700
+++ external_openal/CMakeLists.txt	2018-06-02 12:16:52 -0600
@@ -885,7 +885,8 @@
 OPTION(ALSOFT_REQUIRE_MMDEVAPI "Require MMDevApi backend" OFF)
 IF(HAVE_WINDOWS_H)
     # Check MMSystem backend
-    CHECK_INCLUDE_FILES("windows.h;mmsystem.h" HAVE_MMSYSTEM_H -D_WIN32_WINNT=0x0502)
+    set(CMAKE_REQUIRED_FLAGS "-D_WIN32_WINNT=0x0502")
+    CHECK_INCLUDE_FILES("windows.h;mmsystem.h" HAVE_MMSYSTEM_H)
     IF(HAVE_MMSYSTEM_H)
         CHECK_SHARED_FUNCTION_EXISTS(waveOutOpen "windows.h;mmsystem.h" winmm "" HAVE_LIBWINMM)
         IF(HAVE_LIBWINMM)