In particular,
- Drop unnecessary definition of DESTDIR.
- Drop LIBDIR; pointed to deprecated X11R6 directory.
- Honors CPPFLAGS and LDFLAGS build flags.
- Remove out-of-date version number from installation message.
Based in part on a patch from the Debian package [1].
[1] https://sources.debian.net/src/wmifs/1.4-2/debian/patches/hardening.patch/