pgsql: Enable 64-bit integer datetimes by default, per previous

Log Message:
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Enable 64-bit integer datetimes by default, per previous discussion.
This requires a working 64-bit integer type. If such a type cannot
be found, "--disable-integer-datetimes" can be used to switch
back to the previous floating point-based datetime implementation.

Modified Files:
--------------
    pgsql:
        configure (r1.587 -> r1.588)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/configure?r1=1.587&r2=1.588)
        configure.in (r1.554 -> r1.555)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/configure.in?r1=1.554&r2=1.555)
    pgsql/doc/src/sgml:
        config.sgml (r1.173 -> r1.174)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.173&r2=1.174)
        datatype.sgml (r1.225 -> r1.226)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml?r1=1.225&r2=1.226)
        installation.sgml (r1.305 -> r1.306)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/installation.sgml?r1=1.305&r2=1.306)