tn5250: fix static build with openssl

OpenSSL needs libz. Use pkg-config to find dependencies.

Fixes:
http://autobuild.buildroot.net/results/95f/95f6df0d01e4944ecd84e830e2bbd8389e1d3e3d/
http://autobuild.buildroot.net/results/432/432b1fdec1e6282d51ba1d859c0f4ab9ed2a5efc/
http://autobuild.buildroot.net/results/d6a/d6a7125c21b4f92b69bb9e30b05c4727eedb63ba/



and more.

[Thomas: use += instead of = when assigning <pkg>_CONF_ENV to avoid
future mistakes.]

Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>