Additional Haiku-specific improvements

- Disable checks intended for multi-user systems
  (as Haiku is presently single-user).
- Group Haiku-specific settings in their own section,
  following the pattern for Solaris, OS/2 and AmigaOS
- Add additional library-related settings to remove
  the need for providing configure with custom LDFLAGS.

Submitted by Simon South in #3574538.