build-infra: General permission problems on Windows/cygwin

It seems to be easy to get permission problems on the source code in Windows/cygwin. Some examples:
* If you get the source using TortoiseHG instead of Cygwin hg.
* If you get the source using Cygin hg, but do so in a directory created in the Windows Explorer instead of Cygwin mkdir.
* If you have the source on a mounted network drive.

This results in strange compile problems later on.

We should check in configure that the source code (or at least a few selected files) seems to have proper permissions. This means, at least read permissions for the current user. (A file might be possible to open and read in cygwin even though the "unix" permission does not seem to say so, since that is in fact guarded by Windows ACL which does not always show in the "unix" permissions. However, when copying or processing this file (e.g. for gensrc stuff), the "unix" permissions and not the Windows ACL is what will be used, and we might end up with a file with no read permissions at all, which then will be unreadable).

This also applies to external files we copy in during the build (dll's and possibly others).

Erik Gahlin had a test case of this on his laptop.