[PATCH] INSTALL documentation fixes for 1.8

Hi,

in contrast to the 1.9/1.10 INSTALL patches this one I corrected
existing mistakes/outdates and added a few missing parts. Intentionally
didn't change anything related to supported VS versions or other things
which might be considered inappropriate for 1.8.

[[[
Several small updates/corrections to the INSTALL file:
- case-correction for February
- replaced dead-link to Platform SDK for VC6 with link to still
available instructions
- replaced redirection URL zlib.org with direct URL zlib.net
- libmagic marked as optional dependency
- made URLs consistently use a trailing /
- corrected section numbering and referencing in several cases
- clarify that Python is required for Windows as well
- dropping explicit mentioning of Subversion version 1.8 (since the
documentation always applies to the current version)
- added missing prerequisite an install step instruction for SQLite 3.7.12
- added optional-markers to steps which are not required in all cases
- added missing required quotes around SDK-paths which contain spaces

* INSTALL
     documentation updated
]]]

Regards,
Stefan