Minor issue with "make install" on MS-Windows

[Eli Zaretskii]

I've built Groff 1.21 on MS-Windows using the MinGW tools, and hit a
small snag when installing the result.  The `install' target in the
top-level Makefile ends with this command

          $(LN_S) $(version)$(revision) current

Since native Windows does not support symlinks, LN_S is set to by the
`configure' script "cp -p".  However, "cp -p" by default skips
directories, so this last command fails and the `current' directory is
not produced.

I fixed that by saying "make install LN_S='cp -pr'", but I suggest to
modify the configury to DTRT out of the box in this case.

Thanks!