pgsql: Add support for optional_argument to our own getopt_long() imple

Add support for optional_argument to our own getopt_long() implementation.

07c8651dd91d5a currently causes compilation errors on mscv (and
probably some other) compilers because our getopt_long()
implementation doesn't have support for optional_argument.

Thus implement optional_argument in our fallback implemenation. It's
quite possibly also useful in other cases.

Arguably this needs a configure check for optional_argument, but it
has existed pretty much since getopt_long() was introduced and thus
doesn't seem worth the configure runtime.

Normally I'd would not push a patch this fast, but this allows msvc to
build again and has low risk as only optional_argument behaviour has
changed.

Author: Michael Paquier and Andres Freund

Discussion: CAB7nPqS5VeedSCxrK=QouokbawgGKLpyc1Q++RRFCa_sjcSVrg@mail.gmail.com

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/311da16439ef69fc2054af3f4377fd4acd29a0e3

Modified Files
--------------
src/include/getopt_long.h |    1 +
src/port/getopt_long.c    |   16 ++++++++++++----
2 files changed, 13 insertions(+), 4 deletions(-)