libbarrelfish: Hakefile: fully unify build commands.
libbarrelfish: Hakefile: fully unify build commands.

This commit simplifies the libbarrelfish Hakefile to only state the
sources for libbarrelfish once, and allows selecting #defines and extra
Flounder interfaces per library OS.

TODO: allow inclusion of variant-specific source files and include
paths.

Signed-off-by: Simon Gerber <13ba2e9de9ead0ea0b5f47504fb3a80bb9b1ee3b@inf.ethz.ch>

BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,kishoredbn/barrelfish,kishoredbn/barrelfish,BarrelfishOS/barrelfish,kishoredbn/barrelfish,BarrelfishOS/barrelfish,kishoredbn/barrelfish,kishoredbn/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,kishoredbn/barrelfish,BarrelfishOS/barrelfish