net/mlx: fix meson build with custom dependency path

net/mlx: fix meson build with custom dependency path

If rdma-core is not installed in a standard directory of the system,
it is possible to specify the location of the pkgconfig file via
an environment variable:
PKG_CONFIG_PATH=$PKG_CONFIG_PATH:~/rdma-core/build/lib/pkgconfig

In this case, the dependency may become mandatory to specify
for the configuration tests (checking dependency symbols or fields).

Some spacing is also fixed around.

Fixes: 8e4937640022 ("net/mlx4: add external allocator for Verbs object")
Fixes: 1dd7c7e38c19 ("net/mlx4: support meson build")
Fixes: 96d7c62a70c7 ("net/mlx5: support meson build")
Cc: stable@dpdk.org
Suggested-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Luca Boccassi <bluca@debian.org>
Acked-by: Matan Azrad <matan@mellanox.com>