Fix public issue #37: Only link snappy_unittest against -lz and other autodetected
libraries, not libsnappy.so (which doesn't need any such dependency).

R=csilvers
DELTA=20  (14 added, 0 deleted, 6 changed)


Revision created by MOE tool push_codebase.
MOE_MIGRATION=1710


git-svn-id: https://snappy.googlecode.com/svn/trunk@33 03e5f5b5-db94-4691-08a0-1a8bf15f6143