integrate Android NDK's build flags into this native build system

The Android NDK is also a 'make' build system, with some settings broken
out into easy-to-include files.  This commit makes use of those files to
give both a single file to set certain things, and the native optimized
build flags.

closes #1630
fix setting NDK_UNAME so it only runs the shell script once

`` backticks are not supported by make, they are just passed thru to the
shell.  $(shell) is how make execs stuff in the shell.