[ping] libobjc patches and zlib update

[Matthias Klose]

[CCing some global reviewers]

Please review the two libobjc patches:

  https://gcc.gnu.org/ml/gcc-patches/2016-12/msg02003.html
    - not a regression, but a simplification of the new
      configure option.
  https://gcc.gnu.org/ml/gcc-patches/2016-12/msg02004.html
    - fix the --disable-shared build with external bdw-gc.
      Fixed the texi typo as mentioned by Andreas.

and the proposed zlib update:

  https://gcc.gnu.org/ml/gcc-patches/2017-01/msg00315.html

Thanks, Matthias