Add pkgconfig file

[rossburton]

Now that snowball builds a shared library a pkgconfig file to find the library would be much appreciated.

[ojwb]

Snowball doesn't yet build a shared library (there are patches floating around for it, but nothing in a suitable state for merging yet).

[eli-schwartz]

Snowball also still doesn't install anything... people who are distributing a shared library have to manually cp the library into /usr/lib64 or wherever.

[ojwb]

Snowball also still doesn't install anything

Yes, but that's literally a separate issue (#189) - please don't mix discussions.

[eli-schwartz]

It really is not a separate thing, though. It's a prerequisite. A pkg-config file has to include the path that the library and header file is installed to, it cannot do that unless it is in fact installed.