Don't build the Open Babel GUI

Seems to have issues with the latest wxwidgets and we don't need it.

Change-Id: I2157d0eb15b2584fc2293c3c3b7c52036006c136
Merge branch 'openbabel-tweaks'

Change-Id: I3e2de4438e69a714229b7541d0b547a76e0ec0a5