gecko: convert to CONFIGURE_OUTSOURCE

client.mk is on its way out upstream, switching to it was a mistake.

$ MACH=1 gmake -f client.mk build
client.mk:40: /.mozconfig-client-mk: No such file or directory
gmake: *** No rule to make target '/.mozconfig-client-mk'. Stop.

MFH: 2018Q1 (required by Firefox 59)