MinGW "pwd -W" configure.ac correction

[Glenn Morris]

I think that (thanks to me) there's an issue with the

  test "$MSYSTEM" = "MINGW32" && abs_srcdir=`(cd "$abs_srcdir"; pwd -W | sed -e 
's,^\([[A-Za-z]]\):,/\1,')`

stuff in configure.

Unlike $srcdir (which this used to use), it seems that $abs_srcdir it
not set at this stage in configure. Instead it only gets set by
config.status (IIUC).

Does this `pwd -W' correction only matter for epaths.h?
If so, could it be moved to the epaths-force-w32 rule in Makefile.in?
That rule already does some correcting of paths.

Or does every instance of $abs_srcdir in all the Makefiles need correcting?
If so, we'll have a find way to make config.status do it
(AC_CONFIG_COMMANDS?)...