Add CPPFLAGS and CXXFLAGS to hab-plan-build
Add CPPFLAGS and CXXFLAGS to hab-plan-build

This fixes #926 and #962. If no CPPFLAGS or CXXFLAGS have been set by
your dependent packages (which, as of now, no package has) then they
will become set to the value of CFLAGS.

When you build a new package, it will now set CPPFLAGS and CXXFLAGS
metadata to the value of pkg_include_dirs, just like CFLAGS. That means
as we rebuild packages, they will get new metadata files, and new
builds will honor the metadata rather than draft of CFLAGS.

The side effect is that current packages will Just Work (tm), while
packages that set explicit CPPFLAGS or CXXFLAGS in their plans will
now have the values embedded into their package, making the linking
work correctly.

Signed-off-by: Adam Jacob <adam@chef.io>

Pull request: #965
Approved by: adamhjk

habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat