configure: add configuration options for libcap-ng

configure: add configuration options for libcap-ng

Add configuration option for enabling or disabling linking with
libcap-ng.  Since capabilities are a security feature, the libcapng
option is handled as follows:

    - no option: use libcapng if it's present

    --disable-libcapng: do not use libcapng

    --enable-libcapng: do use libcapng and fail configuration if
                       it's missing

On Linux, not linking with libcapng makes all OVS daemons fail when
--user option is specified.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>