ifdef cleanup

Get rid of a lot of unnecessary ifdefs that just make the code harder to understand and aren't used on the platforms we support today.

• Remove outdated ifdef checks for including stdint.h
• unifdef sequent
• unifdef VAX11C
• Remove ancient Amiga support
• Remove ancient port to 16-bit Windows without X11 libraries
• Remove xpmstrcasecmp fallback for strcasecmp
• Remove xpmstrdup fallback for strdup