Add a -E flag, similar to -e (overide variables from environment) except
that -E only operates for a specified variable. Useful since the -e option
will often pull-in many unwanted variable overrides (esp. in a make world
situation). Uses include overriding BINOWN (which cannot be done by normal
methods or through abuses of MAKEFLAGS) or likely for ports to honour CFLAGS
(provided they're running on a system whose make(1) has this option).