pgsql: Allow MEMSET_LOOP_LIMIT to be set on a per-platform basis, and

Log Message:
-----------
Allow MEMSET_LOOP_LIMIT to be set on a per-platform basis, and turn off
MemSet on AIX by setting MEMSET_LOOP_LIMIT to zero.

Add optimization to skip MemSet tests in MEMSET_LOOP_LIMIT == 0 case and
just call memset() directly.

Modified Files:
--------------
    pgsql:
        configure (r1.473 -> r1.474)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.diff?r1=1.473&r2=1.474)
        configure.in (r1.443 -> r1.444)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in.diff?r1=1.443&r2=1.444)
    pgsql/src/include:
        c.h (r1.194 -> r1.195)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/c.h.diff?r1=1.194&r2=1.195)
        pg_config.h.in (r1.90 -> r1.91)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pg_config.h.in.diff?r1=1.90&r2=1.91)
    pgsql/src/template:
        aix (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/template/aix.diff?r1=1.17&r2=1.18)