configure: New --enable-sparse option to enable sparse checking by default.

configure: New --enable-sparse option to enable sparse checking by default.

Until now, "make" called sparse to do checking only if C=1 was passed on
the command line.  It was easy for developers to forget to specify that.
This commit adds another option: specifying --enable-sparse on the
configure command line enables sparse checking by default.  (It can still
be disabled with C=0.)

Requested-by: Justin Pettit <jpettit@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>