Add script to install build depend for Debian

Add a script debian/deb-build-dep. The script parses debian/control file
and installs the necessary build time dependent packages to the system:

   $ sudo ./debian/deb-build-dep
Debian package: fontforge conflict fontforge-nox

remove sugguestion to install both.
Merge pull request #1121 from abelcheung/master

Fix build for glib < 2.28
Merge pull request #1123 from abelcheung/201401-build-fixes

Disable AT_TESTED in testsuite
Merge pull request #1029 from mozbugbox/install-debian-depends

Add script to install build depend for Debian
Install xdg desktop stuff if xdg utilities are found