Parameterize for CPU architecture, API level, etc.

This enables choosing to build
  - for ARM or x86
  - for a particular Android API level
  - using a particular version of the cross toolchain
by setting a few variables at the top of the build script.

This also allows specifying where to install the NDK and SDK by
setting variables at the top of the build script. The default is to
install them in the $HOME directory.
Remove dependency of glib upon zlib.

Because libz is in the Android NDK.