diff --git a/configure.ac b/configure.ac
index b0c3da15..5ef4ac0b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -99,13 +99,13 @@ AC_SUBST([AVX2_FLAGS])
 
 TEST_AND_ADD_CFLAGS([SSE41_FLAGS], [-msse4.1])
 AS_IF([test -n "$SSE41_FLAGS"], [
-  SAVED_CPPFLAGS=$CPPFLAGS
-  CPPFLAGS="$CPPFLAGS $SSE41_FLAGS"
+  SAVED_CFLAGS=$CFLAGS
+  CFLAGS="$CFLAGS $SSE41_FLAGS"
   AC_CHECK_HEADER([smmintrin.h],
                   [AC_DEFINE(WEBP_HAVE_SSE41, [1],
                    [Set to 1 if SSE4.1 is supported])],
                   [SSE41_FLAGS=""])
-  CPPFLAGS=$SAVED_CPPFLAGS])
+  CFLAGS=$SAVED_CFLAGS])
 AC_SUBST([SSE41_FLAGS])
 
 TEST_AND_ADD_CFLAGS([SSE2_FLAGS], [-msse2])