Make clean before ./configuration.

I ran into the following issue after installing PHP from source and
building the apache libphp5.so module:

```
Restarting web server apache2
apache2: Syntax error on line 185 of /etc/apache2/apache2.conf:
Syntax error on line 1 of /etc/apache2/mods-enabled/php5.load:
Cannot load /usr/lib/apache2/modules/libphp5.so into server:
/usr/lib/apache2/modules/libphp5.so: undefined symbol:
OnUpdateLong
```

As it turns out this can happens under certain circumstances and the
recommended solution is to `make distclean` before attempting to configure
the build.

see: http://serverfault.com/a/148201