emits PKG_CONFIG_PATH; forces re-installation of each explicitly listed package in Aptfile

- avoid `-fPIC` disparity i.e. libvips (Aptfile) + libpango (Cedar 14)

  pkg-config
  libpango-1.0-0
  libpangocairo-1.0-0
  libpangoft2-1.0-0
  libvips-dev
  glib2-dev
  ghostscript
patches LIBRARY_PATH, LD_LIBRARY_PATH to also support i386 / x86_64 specific binaries
adds platform-agnostic variant to PKG_CONFIG_PATH
Merge pull request #6 from evadne/feature/pkg-config

libVips / slugbuilder support