User-specified build flags

[anttikantee]

Make setting BUILDRUMP_C{,PP}FLAGS etc. in the env work as expected: extra flags passed to the compiler.

We probably need two sets of variables: one set hypervisor components and one set for kernel components.

[justincormack]

Now we have the -F option. Any other requirements here?

[anttikantee]

The intention, as far as I remember, was to take them from the env when rumpmake is run. That might just be handled with the buildconfig branch. However, -F doesn't allow separate hypervisor and kernel flags.

[justincormack]

We could add a -h option to just build hypervisor and not kernel perhaps?

[anttikantee]

I don't think it's quite that simple ;-)

The I/O related hypercalls are private to components and generally bundled with them, so some careful planning is required. I'm not quite happy with how virtif users deal with it, though it's the best way that we currently have.

[justincormack]

OK, but that does leave this issue a bit vague ;-)

That reminds me I had another idea about virtif that I was going to experiment with too.

[anttikantee]

I think that we need to come up with a list of scenarios that should work, and then modify the code based on that list. Otherwise we'll just keep modifying the code back and forth.

[justincormack]

OK, well I don't know that I can think of any scenarios where it would be necessary...

[anttikantee]

One easy example: hypercalls should use headers from -I/foo instead of /usr/include