suppress `ar` output for SGC library

The `nicear` script avoids stderr output on DrDr.
build less when a pre-built `racket` is supplied

Adjust dependency tracking and makefile rules to that when
`--enable-racket=...` is provided to `configure`, intermediate
CGC objects are not compiled.

The new approach uses dependency tracking that was already supported
by xform, previously used only for Windows.
racket/draw: avoid an internal error on a bad bitmap
racket/gui: minor test clean-up
fix doc typo
reegxp-match: fix problem with lazy string decoding and output port

Closes PR 14684