Add systemmodule tag & check deps on build (GNOME bug 669554)

Add a systemmodule modtype. systemmodules are modules that must be
provided by your system. 'jhbuild build' will raise an error if the
systemmodule is not provided. systemmodules aim to remove the need for
'sudo yum install libcurl-devel perl-XML-Simple ...' pre-setup.

Use the 'jhbuild sysdeps' command to learn how your system meets the
required dependencies.

To turn off systemmodules, use --nodeps or set in ~/.jhbuildrc:
check_sysdeps = False