Ok i agree it's an enhancement request rather than a bug. I know
environment variables, but my point of view is that /usr/local should
be considered part of default/standard system locations.
I don't think we should suddenly mess with these defaults now,
but there could be an option to add them to CPPFLAGS and LDFLAGS.
Or rather, add $includedir (default /usr/local/include) and $libdir