Thanks for making the changes.
I downloaded again from the CVS fresh and tried:
% autoconf
% ./configure
checking whether make sets $(MAKE)... yes
checking for a BSD-compatible install... /usr/bin/install -c
checking for makeinfo... makeinfo
checking for texi2dvi... false
checking for ptex... ptex
checking for pbibtex... pbibtex
checking for dvipdfmx... dvipdfmx
checking for texi2pdf... false
checking for egrep... egrep
checking for emacs... /cygdrive/c/Program Files (x86)/GNU Emacs 24.5/bin/emacs
checking what a flavor does /cygdrive/c/Program Files (x86)/GNU Emacs 24.5/bin/emacs have... Emacs 24.5
checking prefix for /cygdrive/c/Program Files (x86)/GNU Emacs 24.5/bin/emacs... c:/Program Files (x86)/GNU Emacs 24.5
checking where lisp files should go... $(prefix)/share/emacs/site-lisp/w3m/
checking where icon files should go... $(prefix)/share/emacs/24.5/etc/images/w3m
checking for gzip... /usr/bin/gzip
configure: creating ./config.status
config.status: creating Makefile
config.status: creating doc/Makefile
The process of ./configure went well but 'make.exe' did not go through probably with a similar origin (Windows 10 path of Emacs) of the error: