Allow to build with system liblz4

I plan to submit horde_lz4 to official Fedora repository.

Fedora Guidelines don't allow to use bundle library, see https://fedoraproject.org/wiki/Packaging:No_Bundled_Libraries  Debian have the same Guideline

With this patch applied and with liblz4 r222:

    $ php -d extension=modules/horde_lz4.so /usr/bin/phpunit test
    PHPUnit 4.2.6 by Sebastian Bergmann.
    ............
    Time: 15 ms, Memory: 2.75Mb
    OK (12 tests, 14 assertions)

6c95711 is needed to ensure bundled is not used when building with system version