Add ability to express minimum version numbers for dependencies

[fragglet]

You can do something similar in autotools where you write something like:

PKG_CHECK_MODULES(SAMPLERATE, samplerate >= 0.1.8, [

Currently it's only possible to specify a list of dependencies; you can't say something like:

dependencies "libsamplerate >= 0.1.8"

This is actually kind of important because when we depend on packages which are already installed on the system we use these instead of building our own. But if a dependency is installed but too old we should ignore it.