"make_binary_distribution" does not always generate correct names

Originally, we solved deficiencies of the predefined "autoconf" macros
(at least on OS X 10.5, they do not correctly differ between "x86" and
"x86_64") by providing explicit "--platform" arguments.

With this patch, "make_binary_distribution" evaluates CFLAGS, so it
"just works" because CFLAGS contains information about the target CPU.

This patch is accompanied by a change in our build tools that drops the
setting of "--platform" arguments.