Replace BUILD_DEPENDS by USES=gecko:...,build.

Tested on redports.org, and we have a RUN_DEPENDS on the respective GECKO
application anyways.

Still required to fix upgrade trouble if the gecko application needs a
rebuild from source (ports) if a requisite .so file has gone away, for
instance, after lang/gcc version upgrades.

a RUN_DEPENDS anyhow.


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@368080 35697150-7ecd-e111-bb59-0022644237b5