Merge #11541: Build: Fix Automake warnings when running autogen.sh

cc5c39d [Build] Add AM_OBJCXXFLAGS and QT_PIE_FLAGS to OBJCXXFLAGS to future-proof darwin targets (fanquake)
f8c6697 Fix automake warnings when running autogen.sh (Evan Klitzke)

Pull request description:

  Adjusted @eklitzke's commit to completely remove GZIP_ENV.
  Added a commit to address OBJCXXFLAGS.
  Rebased on master.
  Relevant info from @theuni & #11013 below.

  --------
  GZIP_ENV was indeed added for determinism, but gitian exports this as needed, so it's not really necessary. I'd rather just remove it.

  The mm.o rule was added to support XCode 4.2's ancient version of automake. That's irrelevant now, so it makes sense to remove that too.

  All darwin targets are PIE by default, so we don't technically need the flags, but I'd be more comfortable if we hooked up the OBJCXXFLAGS in case future ones are added.

  --------

  The second commit addresses the last point, but could probably use a better commit message.
  These warnings are removed from autogen output:
  ```
  Makefile.am:12: warning: user variable 'GZIP_ENV' defined here ...
  /usr/local/Cellar/automake/1.15.1/share/automake-1.15/am/distdir.am: ... overrides Automake variable 'GZIP_ENV' defined here
  src/Makefile.am: installing 'build-aux/depcomp'
  src/Makefile.am:503: warning: user target '.mm.o' defined here ...
  /usr/local/Cellar/automake/1.15.1/share/automake-1.15/am/depend2.am: ... overrides Automake target '.mm.o' defined here
  ```

Tree-SHA512: bd59df5f6d3aafe35d5e36925bfe61cc71e774583a0438d7dd946c9e7ecf6e59d42f90a58b8cfef0faa404c81050338ad4cefe721b4a949af881e73b6ab254d4

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