Doc: remove some long-obsolete information from installation.sgml.

Section 16.2 pointed to platform-specific FAQ files that we removed
way back in 8.4.  Section 16.7 contained a bunch of information about
AIX and HPUX bugs that were squashed decades ago, plus discussions of
old compiler versions that are certainly moot now that we require C99
support.  Since we're obviously not maintaining this stuff carefully,
just remove it.  The HPUX sub-section seems like it can go away
entirely, since everything it said that was still applicable was
redundant with material elsewhere in the chapter.

In passing, I couldn't resist the temptation to do a small amount
of copy-editing on nearby text.

Back-patch to v12, since this stuff is surely obsolete in any
branch that requires C99.

Discussion: https://postgr.es/m/15538.1567042743@sss.pgh.pa.us