Added fixes for building Android under Windows

- NDK environment variables are now passed through command line, fixing building under Windows' command line (e.g. using MinGW Makefiles).
- `CMAKE_INSTALL_PREFIX` is no longer overwritten every time CMake runs.
- Installing will now only install static libraries rather than copying object files as well.
- No longer trying to link static libraries with sfml-main, which is
  ignored anyway.