Support building outside of the source tree

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".

Review at https://rbcommons.com/s/OpenH264/r/654/.