configure: add more guidance/reporting on non-recommended options
ffi/unsafe: fix a bug in conversion to UTF-16

Characters outside of the BMP were translated incorrectly.
JIT: fix bug in checking certain values before unboxing

The check was incomplete in the case that both arguments to a binary
[ext]flonum function need to be checked and the second one was not an
[ext]flonum and also not a fixnum.
scribble: preserve order of ".js", ".css", and ".tex" additions