jhbuild: better defaults

["Thomas H.P. Andersen" <phomes gmail com>]

Getting started with jhbuild will be easier if we just have better
defaults. Specifically prefix = /opt/gnome is annoying. We have a few
bugs about the changing the defaults already [1][2][3],
recommendations for overriding the defaults for beginners [4],
complaints [5], and previous discussions here.

> From [1][3][4] the following setup is suggested:

 ~/jhbuild/src          - for checkoutroot
 ~/jhbuild/downloads    - for tarballdir
 ~/jhbuild/mirror       - for dvcs_mirror_dir
 ~/jhbuild/install      - for prefix
 ~/jhbuild/src/jhbuild  - for jhbuild itself (not really a default but
recommended)

Defaulting prefix to a directory under ~ will make jhbuild usable out
of the box. No need to create a .jhbuildrc or fiddle with chmod.

Are there non-bikeshed reasons not to do this? Are there things that
will break so bad that the cost outweigh the benefit?

[1] https://bugzilla.gnome.org/show_bug.cgi?id=655714
[2] https://bugzilla.gnome.org/show_bug.cgi?id=656611
[3] https://bugzilla.gnome.org/show_bug.cgi?id=669439
[4] https://live.gnome.org/Jhbuild/desrt%27sSetup
[5] https://live.gnome.org/AndreasNilsson/PathOfLeastResistance