This makes it clearer which ones actually are real platforms
(chosen automatically or explicitly via "make ARCH=foo") and which
ones just are shared helpers.
This allows building two versions for different architectures at
the same time, without the built files clobbering each other.
This is very helpful when trying to track down differences between
two build configurations.
To build outside of the source tree, create the other directory to
use for building (either outside of the openh264 directory, or as
a subdirectory), enter that directory, and do
"make -f path/to/openh264/Makefile".
Don't enable threading globally since the MSVC/ARM build target
(neither windows phone nor windows rt) doesn't support the normal
windows threading functions.
