roland/systemtap branch

[Roland McGrath]

I used Paul's cute hack to shorten stap-probes.h and I've merged the branch
in.  

I also tweaked the configure support so that --enable-systemtap=auto means
"yes" if a compatible <sys/sdt.h> is found and "no" if not (whereas
--enable-systemtap[=yes] gives a hard error if <sys/sdt.h> is missing or
incompatible).  If we later decide to enable this by default, then all we
have to do is one s/no/auto/ in the AC_ARG_ENABLE for it.

I encourage machine maintainers to add probes to their setjmp/longjmp and
NPTL assembly code, or others to post such patches for other machines.  I'm
happy to assist with getting the LIBC_PROBE uses right in assembly code.

I also encourage Jeff or anyone else who knows of more probe points people
would like added to post patches for those.


Thanks,
Roland