performance enhancements for cygwin make

[Tor Lillqvist]

> as Norbert says I think you may end up with path problems.

Converting between Cygwin and Windows format pathnames is one simple
Cygwin library call, cygwin_conv_path() declared in <sys/cygwin.h>.

>  Incidentally, there is a remake.c (touch_file) method there already,

Weird that it opens the file, reads a byte and writes it back...
What's wrong with utimes() (in the case of an already existing file)?
(Yeah, I guess it isn't equally portable.) But at least in our private
fork, for Windows we definitely should just use SetFileTimeW() on
Windows, perhaps utimes() on Unix, and if that fails because of
non-existent file, then fall back to the portable code that uses
O_CREAT.

> prolly more likely to go up-stream like that.

I expect the biggest hurdle to upstreaming will be "so where are the
unit test additions/changes to check that your changes work and don't
break anything" and "where is the documentation patch" ;)

--tml