tools feature: Undef _GNU_SOURCE at the end of feature tests

Since we get all the tests in a single .c file for a first test,
tools/build/feature/test-all.c, if individual tests set that define and
fail to undef it at its end, then it the test-all.c build will fail due
to defining _GNU_SOURCE multiple times, getting us to the slow path,
so undef it at the end in tests that define it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-w6s00jfo1xabgphzczadl59b@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools build feature: Check if get_current_dir_name() is available

As the namespace support code will use this, which is not available in
some non _GNU_SOURCE libraries such as Android's bionic used in my
container build tests (r12b and r15c at the moment).

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-x56ypm940pwclwu45d7jfj47@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>