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raco setup: fix problem when documentation is not built

Support for pre-rendered documentation could get confused when
the location of source for already-built documentation changes.
add `filesystem-change-evt'

The `filesystem-change-evt' function takes a path to a file
or directory and returns an event that becomes ready when
the file or directory changes (conservatively, so false
positives are possible).

These events are supported on Linux, Mac OS X and other BSD
variants with kqueue(), and Windows.
use `filesystem-change-evt' and static roots to speed library search

Use `filesystem-change-evt' (where supported) to detect changes to
collection link files.

Add "static root" as a new kind of collection root directory in links
files. A static root directory is assumed to not change (in terms of
its collection subdirectories) as long as the links file itself does
not change.

Propagate the notion of static roots through `raco link' and `raco pkg
install'.

Change the `pkg-links' makefile target to install static links
instead of plain links.

The result of all of these changes is to cut 25%-33% of the
time for `racket -l racket', bringing it back in line with
the pre-package-reorganization time.
redex: "info.rkt" fix for moved documentation
minor Makefile and C fixes