Call getmxrr only if it exists

getmxrr is available on Windows sytems from PHP 5.3, whereas concrete5 5.6 requires 5.2.4.
So, don't call getmxrr if it does not exist.
Merge pull request #1849 from mlocati/getmxrr-in-win-from-php-5.3.0

Call getmxrr only if it exists