Add support for building Prox with Meson

Add support for building Prox with Meson

DPDK has deprecated the use of the make system in versions 20.11 and
beyond. Following the deprecation it is non-trivial to keep using make
to build projects that depend on DPDK. A cleaner solution is to adopt
Meson as a building framework. This commit adds support for Meson in
Prox. The meson.build file mirrors the functionality that is available
in the current Makefile. The make build system will exit if it detects
that DPDK was built using meson (testing for the RTE_TARGET directory)

Signed-off-by: Heinrich Kuhn <heinrich.kuhn@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Change-Id: I6ebffa2199993fd6eb46c2f31961fe7dc38e727c