Valid build options for libstemmer

According to the sphinx documentation. The correct build options for compiling libstemmer  are --with-libstemmer not --with-stemmer

> Additional stemmers provided by Snowball project libstemmer library can be enabled at compile time using **--with-libstemmer** configure option. Built-in English and Russian stemmers should be faster than their libstemmer counterparts, but can produce slightly different results, because they are based on an older version.

http://sphinxsearch.com/docs/current.html#installation