Cleaning up some remarkably old stuff in installation.sgml

While poking at the configure options list, I noticed that there
is some other text in installation.sgml that is long past its
sell-by date. For instance, the first para in install-requirements
points to standalone FAQ documents that we removed back in 8.4.
I don't think we need to have blow-by-blow discussions of decades-old
AIX or HPUX bugs either; nor recommendations to use gcc 2.95.3 ;-).
The HPUX section actually seems totally unnecessary after removing
info that is obsolete or duplicative.

Proposed patch attached.

regards, tom lane