Use Autoconf's AC_USE_SYSTEM_EXTENSIONS
This takes care of setting feature test macros (i.e. let Autoconf decide when those can be set safely) to allow subsequent Autoconf tests to better detect available OS features. This also includes a submodule update of unix which enables the use of `AC_USE_SYSTEM_EXTENSIONS` in there as well. Specifically, this takes care of setting `_GNU_SOURCE` (which allows to remove two occurences where it's set manually) and `_ALL_SOURCE` (which fixes issues on AIX). See also https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Posix-Variants.html for details. At some point we may want to reconsider the purpose of "rts/PosixSource.h" and rely more on Autoconf instead.