Express system dependencies using system_deps

This still has a few open tasks:

• Should use a released version of system_deps.
• Should express all system dependencies, even those that are platform dependant. Currently pangoft2 and fontconfig are required unless on win32, and this is only expressed in configure.ac.
• build.sh still has a complex check for libxml-2.0 that should be removed if possible.
• Remove the pkg-config checks from configure.ac as soon as !457 (closed) is merged.

Potentially fixes #675 (closed)