whitespace between '#' and 'cmakedefine' does not seem to work.. at least with my version of cmake-3.9.6.
diff --git a/include/SDL_config.h.cmake b/include/SDL_config.h.cmake
index 673af95..1b644b8 100644
--- a/include/SDL_config.h.cmake
+++ b/include/SDL_config.h.cmake
@@ -232,7 +232,7 @@
# endif
# endif
#else /* non-Apple platforms can use the normal CMake check for this. */
-# cmakedefine HAVE_IMMINTRIN_H 1
+#cmakedefine HAVE_IMMINTRIN_H 1
#endif
#cmakedefine HAVE_LIBUDEV_H 1