net/mlx: fix warnings for unused compiler arguments

net/mlx: fix warnings for unused compiler arguments

When linking the mlx glue code libraries using CC, the linker arguments in
LDFLAGS are not prefixed with -Wl. [The EXTRA_LDFLAGS are though.] This
leads to warning messages on build:

clang-5.0: warning: argument unused during compilation: '-e xport-dynamic'

Fix this by checking for $LINK_USING_CC in the Makefiles and prefixing the
LDFLAGS appropriately if set.

Fixes: 27cea11686ff ("net/mlx4: spawn rdma-core dependency plug-in")
Fixes: 59b91bec12c6 ("net/mlx5: spawn rdma-core dependency plug-in")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>