bug#20858: 25.0.50; Strange messages in the output of 'autogen.sh' (from MSYS2 bash)

[Dani Moncayo <dmoncayo@gmail.com>]

I've observed two strange messages in the output of 'autogen.sh'
(current master branch):

----------------------------------
$ ./autogen.sh
Checking whether you have the necessary tools...
(Read INSTALL.REPO for more details on building Emacs)

Checking for autoconf (need at least version 2.65)...
ok
Checking for automake (need at least version 1.11)...
ok
Your system has the required tools.
Running 'autoreconf -fi -I m4' ...
main::scan_file() called too early to check prototype at
/usr/bin/aclocal-1.11 line 644.
Unescaped left brace in regex is deprecated, passed through in regex;
marked by <-- HERE in m/\${ <-- HERE ([^ \t=:+{}]+)}/ at
/usr/bin/automake-1.11 line 4159.
You can now run './configure'.
----------------------------------

See above the two lines just after "Running 'autoreconf -fi -I m4' ...".

I see those two strange messages from MSYS2 bash, but not from Cygwin
bash.  So I wanted to let you know, in case there is something to fix
(perhaps in my environment?).

TIA.

-- 
Dani Moncayo