libxml2: do not detect zlib/lzma when not available

The host-libxml2 package does not depend on zlib or lzma, but may
detect versions available on the host. This can cause problems in the
following situation:

 1 host-libxml2 is built, finds an available lzma library in the
   system, enables lzma support by using it.

 2 host-xz is built, which installs a different version of the lzma
   library into $(HOST_DIR)

 3 host-libxslt is built, tries to link against libxml2, which fails
   because now the visible lzma library is the one from
   $(HOST_DIR)/usr/lib and no longer the one libxml2 was built agains.

To fix this, this patch passes --without-zlib and --without-lzma to
the host package.

In addition, since the target variant of libxml2 does not have
provisions to depend on the target lzma, we also pass --without-lzma
for the target variant. The case of zlib for the target was already
properly taken into account.

Fixes:

  http://autobuild.buildroot.org/results/6dc/6dcef647eb0f0db7de3f34194eb15ceebc645b39/



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