pgsql: Make it possible, and default, for MingW to build with SSPI

Log Message:
-----------
Make it possible, and default, for MingW to build with SSPI support
by dynamically loading the function that's missing from the MingW
headers and library.

Modified Files:
--------------
    pgsql/src/backend:
        Makefile (r1.122 -> r1.123)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/Makefile.diff?r1=1.122&r2=1.123)
    pgsql/src/backend/libpq:
        auth.c (r1.154 -> r1.155)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c.diff?r1=1.154&r2=1.155)
    pgsql/src/include/port:
        win32.h (r1.74 -> r1.75)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port/win32.h.diff?r1=1.74&r2=1.75)
    pgsql/src/interfaces/libpq:
        Makefile (r1.155 -> r1.156)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/Makefile.diff?r1=1.155&r2=1.156)
        fe-auth.c (r1.130 -> r1.131)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c.diff?r1=1.130&r2=1.131)
    pgsql/src/tools/msvc:
        Solution.pm (r1.30 -> r1.31)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/tools/msvc/Solution.pm.diff?r1=1.30&r2=1.31)