pgsql: Support GSSAPI builds where the header is and not

Log Message:
-----------
Support GSSAPI builds where the header is <gssapi.h> and not <gssapi/gssapi.h>,
such as OpenBSD (possibly all Heimdal).

Stefan Kaltenbrunner

Modified Files:
--------------
    pgsql:
        configure (r1.552 -> r1.553)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.diff?r1=1.552&r2=1.553)
        configure.in (r1.519 -> r1.520)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in.diff?r1=1.519&r2=1.520)
    pgsql/src/backend/libpq:
        auth.c (r1.150 -> r1.151)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c.diff?r1=1.150&r2=1.151)
    pgsql/src/include:
        pg_config.h.in (r1.120 -> r1.121)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pg_config.h.in.diff?r1=1.120&r2=1.121)
    pgsql/src/include/libpq:
        libpq-be.h (r1.59 -> r1.60)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/libpq-be.h.diff?r1=1.59&r2=1.60)
    pgsql/src/interfaces/libpq:
        fe-auth.c (r1.125 -> r1.126)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c.diff?r1=1.125&r2=1.126)
        libpq-int.h (r1.122 -> r1.123)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-int.h.diff?r1=1.122&r2=1.123)