Fix typo

Author: petk

sapi/apache2handler/config.m4

@@ -108,7 +108,7 @@ if test "$PHP_APXS2" != "no"; then
   esac
 
   APACHE_THREADED_MPM=$($APXS_HTTPD -V 2>/dev/null | grep 'threaded:.*yes')
-  AS_VAR_IF_SET([APACHE_THREADED_MPM], [PHP_BUILD_THREAD_SAFE])
+  AS_VAR_SET_IF([APACHE_THREADED_MPM], [PHP_BUILD_THREAD_SAFE])
   AC_MSG_RESULT(yes)
   PHP_SUBST(APXS)
 else