Just checking that the version retrieved from the pkg-config file
matches is not enough. It's nearly tautological since it just checks
that we aren't returning garbage in dep.version(). Instead, check in the
test executable that the pkg-config version retrieved matches the
ZLIB_VERSION exported by zlib.
Also don't require zlib 1.2.8 since RHEL (EPEL) 7 still has 1.2.7 and we
don't really need 1.2.8 anyway.