Changes to sre.c after the application of patch #726869 have increased

Changes to sre.c after the application of patch #726869 have increased
stack usage on FreeBSD, requiring the recursion limit to be lowered
further.  Building with gcc 2.95 (the standard compiler on FreeBSD 4.x)
is now also affected.

The underlying issue is that FreeBSD's pthreads implementation has a
hard-coded 1MB stack size for the initial (or "primary") thread, which
can not be changed without rebuilding libc_r.  Exhausting this stack
results in a bus error.

Building without pthreads (configure --without-threads), or linking
with the port of the Linux pthreads library (aka Linuxthreads) instead
of libc_r, avoids this limitation.

On OS/2, only gcc 3.2 is affected and the stack size is controllable,
so the special handling has been removed.