configure.ac: guard $CFLAGS with "" in tests

Fixes ./configure error:
../mc/configure: line 42709: test: too many arguments

Signed-off-by: Sergei Trofimovich <slyfox@inbox.ru>

configure.ac

@@ -517,7 +517,7 @@ MC_CHECK_CFLAGS
 
 CFLAGS_OPTS=""
 
-if test x$CFLAGS = x; then
+if test "x$CFLAGS" = "x"; then
     CFLAGS_OPTS=" -O2 "
 fi