pkg-infra: make sure cross compiling is enabled when host == target

When compiling for the same architecture and libc as the host,
GNU_TARGET_NAME and GNU_HOST_NAME are equal.  configure scripts use
these to detect cross-compilation, and will decide that we're doing
native compilation.  This may trigger running of executables,
which fail because of missing libraries in the host environment.

To solve this, set the vendor part in GNU_TARGET_NAME to buildroot.

This problem exists for instance in xserver_xorg-server on x86_64.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>