bug#50818: 28.0.50; .eln files need to be executable on Cygwin

[Eli Zaretskii]

> From: Ken Brown <kbrown@cornell.edu>
> Date: Sun, 26 Sep 2021 11:03:53 -0400
> 
> An installed emacs built with native compilation won't start on Cygwin 
> because 
> the executable permissions have been removed from the .eln files.  This is 
> done 
> in the install-eln target of the top-level Makefile.in, which installs the 
> .eln 
> files with 644 permissions even though they were created with executable 
> permissions.

Is what we do different from any other package the installs DLLs?

> Would it hurt anything on other platforms if the .eln files were installed 
> with 
> 755 permissions?  If so, I can submit a patch that does this just on Cygwin.

Please define a new INSTALL_ELN macro for that purpose.

Thanks.