#222 Use pkg-config for finding libxml2

[Jan Tojnar]

src/Makefile.am currently hard-codes the libxml includedir to /usr/include/libxml2. This fails on Nix, which uses different paths. PKG_CHECK_MODULES macro should be used for locating the library.

Upstream PR: https://github.com/NixOS/nixpkgs/pull/38545