package/omniorb: add optional support for zlib

When zlib was compiled before, omniorb will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/lib/libomniZIOP4.so.2.0 | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libomniORB4.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libomnithread.so.4]
 0x0000000000000001 (NEEDED)             Shared library: [libpthread.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libz.so.1]
[...]

The build system offers no option to en-/disable zlib support,
check acinclude.m4, line 435+.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>