use m4_normalize for multiline message

Author: taegyunkim

configure

@@ -1826,9 +1826,9 @@ if test "$Py_OPT" = 'true' ; then
   # Check for conflicting CFLAGS=-O0 and --enable-optimizations
   case "$CFLAGS" in
     *-O0*)
-      AC_MSG_WARN([CFLAGS contains -O0 which may conflict with \
-                   --enable-optimizations. Consider removing -O0 from CFLAGS \
-                   for optimal performance.])
+      AC_MSG_WARN([m4_normalize([
+        CFLAGS contains -O0 which may conflict with --enable-optimizations.
+        Consider removing -O0 from CFLAGS for optimal performance.])])
       ;;
   esac
   # Intentionally not forcing Py_LTO='true' here.  Too many toolchains do not