build: prefer npapi-sdk over mozilla-plugins

npapi-sdk is the broken out version from mozilla. It comes originally
without .pc file, but most dist seem to have added one. In some cases,
also backwards compatible links back to mozilla-plugin (the xul name)
have been added. Others don't want to do that, as xulrunner and
ndpapi-sdk would be conflicting.

With this variant, we can please both cases seen in various distros.