This adds the special .NOTPARALLEL target to include/Makefile when
building on darwin, to work around an apparent bug in the APFS
filesystem.


    PR libstdc++/81797
    * configure.ac (INCLUDE_DIR_NOTPARALLEL): Define.
    * configure: Regenerate.
    * include/Makefile.am (INCLUDE_DIR_NOTPARALLEL): Add .NOTPARALLEL when
    defined.
    * include/Makefile.in: Regenerate.


Tested powerpc64le-linux, and FX reports it works on Darwin. I'll
backport this to the active branches too.