When the ext_dir setting is present, configure php properly for the source recipe

Under CentOS, when php is built from source the extension dir is not set. This results in something like
```
$ php-config --extension-dir
/usr/local/lib/php/extensions/no-debug-non-zts-20100525
```
which means that pecl extensions installed with the `php_pear` resource will not work (ie, the extension.so file will not be copied to the proper directory)