examples: fix make clean when using pkg-config

examples: fix make clean when using pkg-config

The "make clean" command had a number of issues:
- the "--ignore-fail-on-non-empty" flag is not present on BSD
- the call to remove the build folder would fail if there was no build
  folder present.

These are fixed by only removing the build folder if it exists, and by
using -p flag to rmdir in place of --ignore-fail-on-non-empty

Fixes: 22119c4591a0 ("examples: use pkg-config in makefiles")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Luca Boccassi <bluca@debian.org>