fontconfig: tweak font path

Change the font path to include ${prefix}/share/fonts from jhbuild so
that it will find fonts installed by jhbuild (such as Cantarell)

We also look in /usr/local/share/fonts and /usr/share/fonts.

Strictly speaking, we should check all of the XDG_DATA_DIRS, but this
would be a much larger patch.

https://bugzilla.gnome.org/show_bug.cgi?id=720129