build: use the host's build environment by default

We always use gcc from the host, so perhaps we should use the host's
gcc libs by default too. Currently the bare 'make' will fail with
gcc-4.9 because of incompatible gcc libs we have in external/.

See #474.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>