configure: do not set bsd_user/linux_user early

Similar to other optional features, leave the variables empty and compute
the actual value later.  Use the existence of include or source directories
to detect whether an OS or CPU supports respectively bsd-user and linux-user.

For now, BSD user-mode emulation is buildable even on TCI-only
architectures.  This probably will change once safe signals are
brought over from linux-user.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>