mk: build with _GNU_SOURCE defined by default

mk: build with _GNU_SOURCE defined by default

We use _GNU_SOURCE all over the place, but often times we miss
defining it, resulting in broken builds on musl. Rather than
fixing every library's and driver's and application's makefile,
fix it by simply defining _GNU_SOURCE by default for all
builds.

Remove all usages of _GNU_SOURCE in source files and makefiles,
and also fixup a couple of instances of using __USE_GNU instead
of _GNU_SOURCE.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>