Msys2 mingw64 build broken

[Stephen Leake]

As of

   commit d3816bf8ad1fcfed2a32d23216a55850ee4325b5
   Committer : Eli Zaretskii <address@hidden>
   CommitDate: Sat Jul 18 13:02:06 2015 +0300

building master with Msys2 mingw64 is broken. But the problem could be
in an earlier commit, last time I updated was 21 June.

>From 'make bootstrap':

...
make[3]: Leaving directory '/c/Projects/emacs/master-build-mingw64/lisp'
./temacs --batch --load loadup bootstrap
Warning: arch-independent data dir '%emacs_dir%/share/emacs/25.0.50/etc/': 
Permission denied
Warning: Lisp directory 'c': No such file or directory
Warning: Lisp directory 
'C:/msys64//Projects/emacs/master-build-mingw64/../master/lisp': Permission 
denied
Error: charsets directory not found:
c:/Projects/emacs/master-build-mingw64/%emacs_dir%/share/emacs/25.0.50/etc/charsets
Emacs will not function correctly without the character map files.
Please check your installation!


I'd like to learn how to fix such problems; any hints on where to look?
Something to do with handling msys absolute path naming conventions, but
where in the code?

-- 
-- Stephe