scm_cell vs threads build option

[Kevin Ryde]

I gave Rob's new debian packaged 1.8.2 a go and found a bit of a problem
with scm_cell.  The new packages have threads enabled, where the old
ones had it disabled, and alas that setting infects the inlined
scm_cell().  If you built your app against the old and run it against
the new then it bombs on SCM_FREELIST_LOC doing the different "thread
key" thingie access.

I guess scm_cell has been inlined that way for a while, but it'd be
worth thinking about not inlining it, or only inlining for internal
uses, in the interests of binary compatibility among as many build
options as possible.

I struck the problem in my own build of guile-gtk, not sure what
packaged stuff might be affected.  gnome-games seems ok.  Anything using
smobs probably isn't.