fix a typo in gbuild help and improve a autoconf error message

Change-Id: Iaee342ae6d612e35d1d2dfa91edbc952d4f73b4e
Reviewed-on: https://gerrit.libreoffice.org/11616
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>