Log Message:
lang/gprolog: Build failure is not jobs safe

gprolog fails to build on DragonFly-*-i386.
A fatal error (exception raised) occurs on fd2c.pl:215-220 on this
platform.  The build hangs after that (halts bulk build forever),
another job apparently never exits.

Setting the package to a single job fixes the problem, so apparently
a successful build is jobs safe, but not a build failure.