Restore build without __sync_fetch_and_{add,sub} (re: 07cc71b8)

The more I think about it, the more it seems obvious that commit
07cc71b8 (PR #14) is quite simply a workaround for a GCC optimiser
bug, and (who knows?) possibly an old, long-fixed one, as the bug
report is years old.

The commit also caused ksh to fail to build on HP-UX B.11.11 with
GCC 4.2.3 (hosted at polarhome.com), because it doesn't have
__sync_fetch_and_add() and __sync_fetch_and_sub(). It may fail on
other systems. The GCC documentation says these are legacy:
https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html

HELP WANTED: what I would like best is if someone could come up
with some way of detecting this optimiser bug and then error out
with a message along the lines of "please upgrade your broken
compiler". It would probably need to be a new iffe test.

Meanwhile, let's try it this way for a while and see what happens:

src/cmd/ksh93/include/jobs.h:
- Restore original ksh version of job_lock()/job_unlock() macros.
- Use the workaround version only if the compiler has the builtins
  __sync_fetch_and_add() and __sync_fetch_and_sub().

TODO

@@ -9,7 +9,7 @@ Fix regression test failures:
 ______
 Fix build system:
 
-- ksh does not currently build on NetBSD, AIX, HP-UX, Solaris, or QNX.
+- ksh does not currently build on NetBSD, AIX, Solaris, or QNX.
 - Reimport the removed nmake. It is necessary for changes in Makefiles
   to take effect. The machine-generated Mamfiles are now used as a fallback,
   but they are not meant to be edited by hand.