package/xerces: add optional support for icu

When icu was compiled before, xerces will use it as optional dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/lib/libxerces-c-3.1.so | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libgcc_s.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libicudata.so.56]
 0x0000000000000001 (NEEDED)             Shared library: [libicui18n.so.56]
 0x0000000000000001 (NEEDED)             Shared library: [libicuuc.so.56]
 0x0000000000000001 (NEEDED)             Shared library: [libm.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libstdc++.so.6]

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