build/nmk: simplify try-cc and return true/false

- simplify: don't use temporary file -- use /dev/null instead
- return 'true' or 'false' -- this way we can use it inside rules
without external call to bash to compare return with 'y', see
the next patch for a use case.

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>