avoiding SEGV during SMOB GC with gc 7.3

[Mike Gran]

Hi-

There's something we discussed in bug report #13611 that
might be of general interest.

If you're using a gc built with --enable-parallel-marks
with Guile, you can get a SEGV during garbage collection
of a SMOB.  BDW-GC v7.3 has that flag enabled by default.
It allows the garbage collector to spawn a new thread
to mark data.


Also, Fedora has that flag enabled by default with v7.2.

This only applies if you run Guile on a multi-core system.


It appears you can disable this behavior, even if you
compiled GC with --enable-parallel-marks, by 

setenv GC_MARKERS to 0 or 1.

-Mike