Add a patch for memcached to ccache along with a slave devel/ccache-memcached port.

This patch is not safe for WITH_CCACHE_BUILD support yet as that causes all
ports to depend on devel/ccache. Enabling that patch would then cause the
new devel/libmemcached dependency to require devel/ccache which is a cyclic
dependency. The autoconf dependency also causes issues.

Add a devel/ccache-memcached slave port that would allow a user to use
the ccache+memcached package manually with ports without WITH_CCACHE_BUILD.

This patch comes from https://github.com/ccache/ccache/pull/58 and has been
an ongoing effort over a few years to be merged into the mainline of ccache.
Documenation for it can be found in the MANUAL file at:

/usr/local/share/doc/ccache/MANUAL.txt

Sponsored by: Dell EMC Isilon