pgsql: Enable building with the Mingw64 compiler.

Enable building with the Mingw64 compiler.

This can be used to build 64 bit Windows binaries, not only on 64 bit
Windows but on supported cross-compiling hosts including 32 bit Windows,
Cygwin, Darwin and Linux.

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=91812df4ed0facfb90bec3f9430dd5a97d56f695

Modified Files
--------------
config/ac_func_accept_argtypes.m4   |    5 +++--
configure                           |    4 ++--
src/include/c.h                     |    2 +-
src/include/port.h                  |    4 ++++
src/include/port/win32.h            |   18 +++++++++++++++++-
src/include/port/win32/sys/socket.h |    1 +
src/port/getaddrinfo.c              |    3 +--
src/test/regress/resultmap          |    3 +++
8 files changed, 32 insertions(+), 8 deletions(-)