gauche: explicitly handle zlib

host-gauche currently fails to build if host-zlib was built before,
because it finds zlib.h, but due to issues in gauche's build system,
the LDFLAGS passed in the configure script (with -L
$(HOST_DIR)/usr/lib) are not passed all the way down to where -lz is
used, causing a build failure.

Since we don't need zlib support in host-gauche, we simply disable it
explicitly.

While we're at it, we make the optional dependency on zlib explicit
for the target variant of the gauche package.

Fixes:

   http://autobuild.buildroot.org/results/426/4269c465312ddcc801289914fa29427798ef7783/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>