configure fixes for Tru64 UNIX V4.0x

[Sergio Gelato]

1) Testing of uidswap.c on a Tru64 UNIX V4.0G PK4 (BL22) machine shows the
following defines to be required for correct uid changing semantics:

#define BROKEN_SETREGID 1
#define BROKEN_SETREUID 1
#define SETEUID_BREAKS_SETUID 1

Failure to fix these contributes to breaking privilege separation
(in a safe way: connections will fail while UsePrivilegeSeparation=yes,
thanks to permanently_set_uid()'s built-in sanity checks).

I don't know what the situation is on later (V5.x) releases of Tru64.
But for $host matching *-dec-osf4.0* the above settings should help.

2) The configure script as distributed with 3.7.1p1 has some cpp directives
with leading space before the #. The /usr/bin/cc bundled with Tru64 V4.0G
chokes on these unless an explicit -std option (e.g., cc -std1) is given.
Unfortunately, turning on this option creates other compilation problems.

It would be safest to make all cpp directives start with the # in column 1,
since that's more portable.

3) I'm also working on auth-sia.c fixes; will submit them once I'm satisfied
that they work. My interest is in the BSD and Heimdal (krb5) modules; I'm
unfortunately not in a position to test with the OSFC2 module.