package/tor: add optional support for libcap

package/tor: add optional support for libcap

- without libcap
$ host/usr/bin/x86_64-linux-readelf -a target/usr/bin/tor | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libevent-2.0.so.5]
 0x0000000000000001 (NEEDED)             Shared library: [libssl.so.1.0.0]
 0x0000000000000001 (NEEDED)             Shared library: [libcrypto.so.1.0.0]
 0x0000000000000001 (NEEDED)             Shared library: [libz.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.0]

- with libcap
$ host/usr/bin/x86_64-linux-readelf -a target/usr/bin/tor | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libevent-2.0.so.5]
 0x0000000000000001 (NEEDED)             Shared library: [libssl.so.1.0.0]
 0x0000000000000001 (NEEDED)             Shared library: [libcrypto.so.1.0.0]
 0x0000000000000001 (NEEDED)             Shared library: [libz.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libcap.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.0]

The tor configure script has no option to en-/disable libcap support.

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