fix cross compile on QNX

got broken in 2e284cc78357ccc4a63aae7603682b0344bfc58f
The racket version of libunwind is not compatible with QNX but old-style stacktraces are still working with the default gcc version
Slight change to how drracket handles lexer/colorers, and doc changes to color:text<%>
add slide-pict/center
update dns tests based on change at nwu
Mac OS X PPC: fix dylib paths

Relevant to PR 14694
optimizer: more optimizations for unary operations

Previously, the optimizer simplified the application of some unary functions inside let,
for example (car (let () ... (cons 1 2)) => (let () ... 1). This commit extends this to begin forms,
like (car (begin ... (cons 1 2)) => (begin ... 1).

Also, constant folding and some reductions were only availed in the direct case, for example
(procedure? car) => #t. With this commit these reductions are extended to the expressions
inside let and begin, for example (procedure? (let () (begin ... car))) => (let () (begin ... #t).