add --with-system-fastlz option

Sorry to bother you with this change...

But Fedora Guidelines want "No bundled library".
As pecl/memcached, pecl/yac, pecl/couchbase, php-fastlz... use it, It make sense to have it shared.

This change should be "noop" for standard build.

P.S. also see https://github.com/php-memcached-dev/php-memcached/pull/151 and https://github.com/laruence/yac/pull/42