net/mlx5: support meson build

net/mlx5: support meson build

Compile Mellanox driver when its external dependencies are met.  A
glue version of the driver can still be requested by using the
-Denable_driver_mlx_glue=true

Meson will try to find the required external libraries.  When they are
not installed system wide, they can be provided though CFLAGS, LDFLAGS
and LD_LIBRARY_PATH environment variables, example (considering
RDMA-Core is installed in /tmp/rdma-core):

 # CLFAGS=-I/tmp/rdma-core/build/include \
   LDFLAGS=-L/tmp/rdma-core/build/lib \
   LD_LIBRARY_PATH=/tmp/rdma-core/build/lib \
   meson output
 # LD_LIBRARY_PATH=/tmp/rdma-core/build/lib \
   ninja -C output install

Note: LD_LIBRARY_PATH before ninja is necessary when the meson
configuration has changed (e.g. meson configure has been called), in
such situation the LD_LIBRARY_PATH is necessary to invoke the
autoconfiguration script.

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>