We have to pass the path to gcc when calling windres, which itself
might have spaces in.  Furthermore, we have to pass the path to gcc's
tools to gcc.  This means getting the quoting right, and after much
experimentation and reading of the windres sources I found something
that works: passing --use-temp-files to windres makes it use its own
implementation of quoting instead of popen(), and this does what we
want.  Sigh.