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
build native code for armeabi-v7a, older devices can use Java OpenPGP

building for armeabi-v7a could give a dramatic speed boost to armv7a CPUs,
which have been the standard for Android devices for a while.  Since there
are Java OpenPGP implementations available, we don't need to worry so much
about supporting the very old or very cheap hardware that does not include
armv7a CPUs.  This has become more important given all the ARM assembly
added to libgcrypt recently.

refs #1630