bug#69994: 30.0.50; Issue building on msys

[Ergus]

Hi:

I am trying to build master on msys and I get this error:


```
git clone ....
cd Emacs
./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
Your system has the required tools.
Running 'autoreconf -fi -I m4' ...
ac-wrapper: autoreconf: warning: auto-detected versions not found ( ); falling 
back to latest available
' is already registered with AC_CONFIG_FILES.
../autoconf-2.72/lib/autoconf/status.m4:289: AC_CONFIG_FILES is expanded from...
configure.ac:7795: the top level
autom4te-2.72: error: /usr/bin/m4 failed with exit status: 1
autoreconf-2.72: error: /usr/bin/autoconf-2.72 failed with exit status: 1
```

I have already build emacs in this same system with no issues.
Any idea?

Best,
Ergus