atomic64 and python modules

[Greg Troxel <gdt%lexort.com@localhost>]

I'm following up on bulk build failures for py-gdal on i386.

Basically, gdal needs 64-bit atomics, and is mostly used in practice on
x86_64 and aarch64.  I've added:

  .include "../../mk/atomic64.mk"

to the gdal-lib Makefile, and that works -- gdal-lib builds on i386.

py-gdal has the same, but the build errors out:

  -- Performing Test HAVE_SHARED_MUTEX
  -- Performing Test HAVE_SHARED_MUTEX - Success
  -- Performing Test HAVE_ATOMIC_UINT64_T
  -- Performing Test HAVE_ATOMIC_UINT64_T - Failed
  -- Performing Test HAVE_ATOMIC_UINT64_T_WITH_ATOMIC
  -- Performing Test HAVE_ATOMIC_UINT64_T_WITH_ATOMIC - Failed
  CMake Error at cmake/helpers/configure.cmake:431 (message):
    Cannot find libatomic needed to have support for std::atomic<uint64_t>
  Call Stack (most recent call first):
    gdal.cmake:70 (include)
    CMakeLists.txt:264 (include)

Reading atomic64.mk, it adds -march=i586 to CFLAGS.  Apparently that
isn't propagating from a BSD make variable through setup.py and
wrappers.   I don't see that in .cwrappers/config.

Does atomic64 work with python wheel.mk?  Any other clues to debug?

(Personally I don't use i386 for much of anything except a bit of
old-school server programs, so this doesn't bother me.  But I am trying
to follow up on bulk failure reports.)