pgsql: Enable GSSAPI to build using MSVC.

Log Message:
-----------
Enable GSSAPI to build using MSVC. Always build GSSAPI when Kerberos is
enabled, because the only Kerberos library supported always contains it.

Modified Files:
--------------
    pgsql/src/backend/libpq:
        auth.c (r1.151 -> r1.152)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c.diff?r1=1.151&r2=1.152)
    pgsql/src/include/libpq:
        libpq-be.h (r1.60 -> r1.61)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/libpq-be.h.diff?r1=1.60&r2=1.61)
    pgsql/src/interfaces/libpq:
        fe-auth.c (r1.126 -> r1.127)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c.diff?r1=1.126&r2=1.127)
    pgsql/src/tools/msvc:
        Solution.pm (r1.28 -> r1.29)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/tools/msvc/Solution.pm.diff?r1=1.28&r2=1.29)