meson: move zlib detection to meson

Meson includes the same logic that tries to look for -lz if
pkg-config (and cmake) cannot find zlib.  The undocumented
--disable-zlib-test option becomes a no-op.

There is still an instance of "-lz" in the LIBS directory.
It will go away as soon as tests are converted to meson,
because the zlib dependency does not propagate from libblock.fa
to the Makefile-build unit tests.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>