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.