pgsql: Support platforms where strtoll/strtoull are spelled__strtoll/_

Support platforms where strtoll/strtoull are spelled __strtoll/__strtoull.

Ancient HPUX, for one, does this.  We hadn't noticed due to the lack
of regression tests that required a working strtoll.

(I was slightly tempted to remove the other historical spelling,
strto[u]q, since it seems we have no buildfarm members testing that case.
But I refrained.)

Discussion: https://postgr.es/m/151935568942.1461.14623890240535309745@wrigleys.postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/06f66cff9e0b93db81db1595156b2aff8ba1786e

Modified Files
--------------
configure                     |  4 ++--
configure.in                  |  4 ++--
src/include/c.h               | 47 ++++++++++++++++++++++++++-----------------
src/include/pg_config.h.in    |  6 ++++++
src/include/pg_config.h.win32 |  6 ------
5 files changed, 39 insertions(+), 28 deletions(-)