install libs built Android-style into libs/$(APP_ABI)/ instead of assets/

This means that Android handles the rest for us, including bundling and
unpacking them from the APK, setting the LD_LIBRARY_PATH, etc.

refs #1548
REBASE add Android support commit to m4/libtool.m4 on any project that uses it
REBASE more reliable integration of Android NDK build files into external/Makefile
update npth to latest in master

closes #1644