Look for libxml2 via pkg-config first

If no explicit path is specified, try pkg-config first, before
xml2-config. The reason is that pkg-config knows the difference between
static and shared dependencies and thus doesn't cause libxslt to be
linked against a bunch of extra stuff.

Say for example that libxml2 is compiled --with-icu, then it will be
linked against various libicu shared libraries. xml2-config will _also_
specify those libraries (because it doesn't know whether you are doing
static or shared linking) and thus libxslt is also linked against libicu
even though it does not use it.

On the other hand, pkg-config has Libs/Libs.private which separates
shared&static dependencies and so you can get libxslt to link to _only_
libxml2 without any other things.

Fixes bug #778549:

    https://bugzilla.gnome.org/show_bug.cgi?id=778549