Change how we compute install paths on cygwin

We used to have
    MK_INSTALL_DEST = "$(shell cygpath $1)"
but this meant we ended up with
    "$(shell cygpath "[...]/html/`basename $$i`")"
and the $(...) gets evaluated before the makefile rule, so the for loop
hasn't been run, and so $i isn't defined. So we were taking the basename
of the empty string, meaning docs weren't being installed in the right
place.

Now we have
    MK_INSTALL_DEST = $$(cygpath $1)
so the evaluation happens in the shell, while the for loop is running.