Log Message:
Fix a problem with MAKE_VERSION being undefined on case-insentive filesystems.

On case-insensitive filesystems such as MacOS, two files (Makefile and makefile,
which is created by configure from makefile.in) collide.  Only the former
defines MAKE_VERSION, but it is replaced by the latter.  Consequently,
MAKE_VERSION is defined as an empty string on these systems.  This patch is
the result of a discussion on tech-pkg (see the thread following
http://mail-index.netbsd.org/tech-pkg/2017/08/13/msg018629.html) and is
based upon code incorporated into bmake v20170812.