x11-wm/openbox: Remove PANGO option and make it mandatory

PANGO_CONFIGURE_OFF was spelled incorrectly, but --disable-pango
does not work:

configure: WARNING: unrecognized options: --disable-pango

Upstream has made it mandatory 13 years ago in
https://github.com/Mikachu/openbox/commit/037fc862b0dd4535accf7b69a8c0112355bd04b1