- Fix stack overflow detection algorithm. It has not worked before as
we were linking the ruby binary against pthreads, and the default
stack size detection method with getrlimit didn't returned right
values in this case. Now, if threads enabled, it also tries to
determine the stack size via pthreads calls and use this value if
it is smaller than what getrlimit returned. Furthermore, the stack
overflow detection routine now works proactively, generating
exception if there're probability the stack will be exhausted by
the time of the next check (ruby performs checks only in each 256th
call of rb_call0). 
- Build pthreads-enabled ruby by default. I have not received any
bug reports for this for years, and this verison will work correctly
with threaded libraries. Also, do not link agains pthreads in non-pthread
case (this breaks stack size detection algorithm), and eliminate the
option to disable pthreads (so only power users who know what they're
doing can disable them).
- Build RDoc by default so it is available in the package.
- Bump portrevision.
Reported by: Eugene Pimenov <firstname.lastname@example.org>