Create a dynamic library.

As pointed out by Adam Borowski, static linking makes updates tedious.
This patch switches the package over to create a dynamic library
instead.

We have to specify -fPIC for compiling and --shared for linking. The
shared object is called liblopsub.so.X.Y.Z, where X, Y, and Z are
the digits of the version string as created from the git revision by
the version-gen.sh script. The soname for the linker to put into each
executable which is linked against lopsub is liblopsub.so.$X.

The install target is modified to install liblopsub.so.X.Y.Z instead
of liblopsub.a, and it also creates the usual symbolic links for
shared libraries. It does not run ldconfig, though.

Requested-by: Adam Borowski <kilobyte@angband.pl>