remove spaces and tabs where 'make' complains about them

spaces and tabs can have meaning in a Makefile, so stray ones can cause
troubles.  emacs makefile-mode warns about potentially troublesome errant
whitespace.
include note about git submodules in BUILD instructions
update native build to include Android NDK build flags

This updates external/Makefile to be more integrated into the Android NDK,
by using the same variable names and including build flags from the NDK.