package/libeXosip2: add optional support for c-ares

package/libeXosip2: add optional support for c-ares

When c-ares was compiled before, libeXosip2 will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/sip_reg | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [libcares.so.2]
[...]

The build system offers no option to en-/disable c-ares support:
http://git.savannah.gnu.org/cgit/exosip.git/tree/configure.in?h=exosip-3.X.X#n208

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