Bug#702011: pangox support in gtkmathview

[Luca Padovani]

Unfortunately I'm no longer able to maintain gtkmathview.

In principle pangox is just *one* of the supported backends and, if it is not found when ./configuring gtkmathview, the gtk backend should be selected and used instead. In practice it's a bit hard for me to predict whether this would compromise the use of gtkmathview, especially when used in conjunction with host applications (AbiWord) as I'm not up-to-date on development on Debian/Linux platforms.