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.
Merge pull request #31 from MarioLiebisch/build-fixes

Added fixes for building Android under Windows