Add support for multi-platform builds

`./build.sh` is now the default entry point into building libopencash. Cmake
seemed limited and cumbersome for the type of functionality required by
multi-platform and universal builds. Going with an external shell script is
less portable but saved some headache for now.

Currently `./build.sh` automates the following tasks:

  - building for multiple platforms,
  - aggregating static libs into a single lib archive,
  - and creating universal lib archives for the platforms 'ios-universal'
    'osx-ios-universal'.

In cmake, OC_TARGET was renamed to OC_PLATFORM. That seemed like a more
appropriate word to represent the combination of target architecture and OS.