new jhbuild features: jhbuild sysdeps and partial_build = True

[Colin Walters <walters verbum org>]

Hi GNOME developers,

So for a long time, jhbuild has always looked in your system for
pkg-config dependencies.  However, if the moduleset said to build
something, we'd still build it.

Concretely for example, if the moduleset said to build dbus, we'd
build it, even if your system had a new enough version.  People who
wanted to build "just my module" have historically been able to do
things like skip = ['dbus', ...].

In the latest jhbuild, when the "partial_build" key is set (the
default) we explicitly look at what's installed on the system, and if
they're new enough, omit them from the build list (unless you have
built them before).

You can better understand what jhbuild is doing by looking at the
output of the new "sysdeps" command, like this:

$ jhbuild sysdeps
System installed packages which are new enough:
  speex (required=1.2rc1, installed=1.2rc1)
  libdiscid (required=0.2.2, installed=0.2.2)
  telepathy-farsight (required=0.0.14, installed=0.0.16)
  lcms2 (required=2.2, installed=2.2)
...
System installed packages which are too old:
  webkitgtk-3.0 (required=1.5.1, installed=1.4.0)
  ...
No matching system package installed:
  libvala-0.14 (required=0.13.1)
  ...

Now, when you do e.g. "jhbuild build" or just "jhbuild list", we will
omit modules in the first list (again, unless you've built them
before).

If you're starting from scratch with a recent distribution, this can
easily drop 50 or more modules from the list of things to build.
Enjoy!