On FreeBSD liblz4 is installed in /usr/local/lib.
Groonga uses pkg_check_modules to check for liblz4 (that is, pkg-config),
and then it used to set for libgroonga.a
link_directories({$LIBLZ4_LIBRARY_DIRS})
target_link_libraries(... ${LIBLZ4_LIBRARIES})
Now groonga is a static library, linked into ha_mroonga.so. CMake won't
link dynamic liblz4.so into libgroonga.a, instead it'll pass it as a
dependency and will link it into ha_mroonga.so. Fine so far. But it will
not pass link_directories from the static library as a dependency,
so ha_mroonga.so won't find liblz4.so
As suggested on cmake mailing list (e.g.
here: http://public.kitware.com/pipermail/cmake/2011-November/047468.html)
we switch to use the full path to liblz4.so, instead of the -l/-L pair.
