package/avahi: add optional support for libcap

When libcap was compiled before, avahi daemon will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/sbin/avahi-daemon | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [libavahi-common.so.3]
 0x0000000000000001 (NEEDED)             Shared library: [libavahi-core.so.7]
 0x0000000000000001 (NEEDED)             Shared library: [libcap.so.2]
[...]

The build system offers no option to en-/disable libcap support:
http://git.0pointer.net/avahi.git/tree/configure.ac#n382



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: make the test unconditionally, even if libcap is only used by
avahi-daemon for the moment.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>