implement fallback download for php-src

[henderkes]

php.net is constantly down, we should fall back to at least github sources, or mirror php.net

[crazywhalecc]

php-src is a special source, we have minor version selection. For mirror in v2, I think it should be done in PhpSource.php, and v3 will need another php-release-mirror type.

Another issue is the version query API. We cannot use API as well, unless hosting https://github.com/php/web-php-distributions and caching web-php API ourself.

[crazywhalecc]

BTW it seems gmplib.org has banned GitHub IP forever. We need to change it to ftp.gnu.org or any other mirror site.