Be able to compile YACassandra statically into PHP

Compilation of YACassandraPDO fail if you compile it statically inside php (http://www.php.net/manual/en/install.pecl.static.php).
This patch allow compilation of php via : 
./buildconf --force
./configure --enable-pdo --with-pdo-cassandra=ext/pdo_cassandra/interface/cassandra.thrift

Readme should be updated too to add the correct command line.
I'm not a m4 expert, so maybe there is a better solution instead of adding  --with-pdo-cassandra=ext/pdo_cassandra/interface/cassandra.thrift

Note that folder need to be named "pdo_cassandra" (e.g phpsource-5.4.x/ext/pdo_cassandra/).