pgsql: A few fixups in error handling: mark pg_re_throw() as noreturn

Log Message:
-----------
A few fixups in error handling: mark pg_re_throw() as noreturn for gcc,
and for other compilers, insert a dummy exit() call so that they understand
PG_RE_THROW() doesn't return. Insert fflush(stderr) in ExceptionalCondition,
per recent buildfarm evidence that that might not happen automatically on some
platforms. And const-ify ExceptionalCondition's declaration while at it.

Modified Files:
--------------
pgsql/src/backend/utils/error:
assert.c (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/assert.c.diff?r1=1.33&r2=1.34)
elog.c (r1.184 -> r1.185)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c.diff?r1=1.184&r2=1.185)
pgsql/src/include:
postgres.h (r1.79 -> r1.80)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/postgres.h.diff?r1=1.79&r2=1.80)
pgsql/src/include/utils:
elog.h (r1.85 -> r1.86)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/elog.h.diff?r1=1.85&r2=1.86)