tests/lib/c_lib: Set standard source macro appropriately

strnlen() and strtok_r() are tested in this files
which are extensions to the the
std C library. Let's explicity select one of the extensions
also when building with the host C library,
instead of relaying on somebody having
set it for this file somewhere else.

Also, let's be nice and undefine them first in case they had
been defined somewhere else in the build scripts.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>