[x265] SONAME and shared lib install

[Derek Buitenhuis]

Only sending this to here because it's very hard to catch Steve on IRC,
since I'm in GMT.

In the process of hooking up x265 to Libav/FFmpeg, I've noticed a number
of deficiencies in x265's cmake stuff:

1) Shared library is never install.
2) No pkg-config file is generated or installed, despite
   x265.pc.in existing.
3) No SONAME on the shared library.

Before I can feasibly send the patch to Libav/FFmpeg, at the very least,
the former two must be fixed, so it can be integrated with others'
configure scripts properly in a platform-agnostic manner.

The wrapper itself is done, and this is all that remains.

- Derek