RMAIL issue on Windows build

[Angelo Graziosi]

Usually I backup my mails with 'archivemail' which saves them as mbox. Then I 
can read them with RMAIL. If the mbox archive is 'Sent_box', I can read it with

M-x rmail
i Sen<TAB>  (completed in 'Sent_box')
h

At this point it shows a list of mails which I can select and read.

I did this both on Windows and GNU/Linux.

Now, with my last builds of master (20190802_231128), the above steps do not 
work on Windows (MSYS2/MINGW64),

i Sen<TAB>

does not complete (says: don't match). Even copy/pasting the full archive name 
(Sent_box), it seems to load an empty buffer..

On both systems the builds were configured '--without-pop', only the on Windows 
the build log says

[...]
config.status: executing src/epaths.h commands
config.status: executing src/.gdbinit commands
config.status: executing doc/emacs/emacsver.texi commands
config.status: executing etc-refcards-emacsver.tex commands

 ===> Building ...

make -C nt all
make -C lib all
make info-real info-dir
make[1]: ingresso nella directory "/tmp/emacs-master/nt"
  RC       emacs.res
  CCLD     addpm.exe
  CCLD     cmdproxy.exe
[...]
-------------------

while on GNU/Linux

[...]
config.status: executing src/epaths.h commands
config.status: executing src/.gdbinit commands
config.status: executing doc/emacs/emacsver.texi commands
config.status: executing etc-refcards-emacsver.tex commands
configure: You might want to install GNU Mailutils
<https://mailutils.org> and use './configure --with-mailutils'.

 ===> Building ...

make -C lib all
make info-real info-dir
make[1]: ingresso nella directory "/tmp/emacs-master"
make[1]: ingresso nella directory "/tmp/emacs-master/lib"
make -C doc/lispref info
[...]

Ciao,
  Angelo.