build: remove redundant _GNU_SOURCE definitions

build: remove redundant _GNU_SOURCE definitions

The feature macro _GNU_SOURCE is defined globally,
but there was some remaining useless settings.

The internal definition in config/meson.build is kept,
all other internal definitions of _GNU_SOURCE are removed,
except in examples, which can be built as external applications.
Note: external applications do not inherit of _GNU_SOURCE.

Fixes: 5d7b673d5fd6 ("mk: build with _GNU_SOURCE defined by default")
Fixes: 28188cee2aa0 ("build: enable BSD features visibility for FreeBSD")
Fixes: e6cdc54cc0ef ("net/mlx5: add socket server for external tools")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: David Marchand <david.marchand@redhat.com>