deskutils/subsurface: Fix build when building with a single core

It happens that when building the port with poudriere without
ALLOW_MAKE_JOBS=yes, the build fail. I received similar failure reports
from pkg-fallout but could not reproduce them until today where
rebuilding llvm on my laptop required me to remove ALLOW_MAKE_JOBS=yes
before I had to rebuild subsurface.

Setting MAKE_JOBS_UNSAFE=yes allowed me to reliably reproduce the
failure, and relying on GNU make instead of BSD make prevent it.

So use gmake to build subsurface and hopefully say goodbye to the
pkg-fallout failure notifications \o/.