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
AT_TESTED only checks for programs in existing path. Not only is this unuseful in test suite, it actually causes the whole test suite fail to run under clean environments (such as chroot)
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