enable build for shared library

Hello Andrej. As I already wrote (by mail), I need libpkgconf shared library (You did not answer). So I make this merge request to enable building shared library. Please merge it or write me what need to be fixed.

Probably debian/changelog must be fixed (I set there my e-mail).

Also static library and headers need to be splitted in -dev package. If You want - I can do this.

-- Alex Syrnikov (Леша Сырников)