Cmake improvements

Adds several improvements to CMake:
  - 'emconfigure cmake' will now use the provided Emscripten toolchain file instead of creating one on the fly. Closes #1464. Helps with #696, #695 and #681, although those will need more work.
  - Tell CMake explicitly that we are cross-compiling, and avoid CMake to try to compile and run a test program, since it will fail because the output is not natively executable.
  - Simplify CMake handling by not needing separate Emscripten.cmake and Emscripten_unix.cmake. 
  - Simplify CMake handling by not needing to pass CMAKE_MODULE_PATH on the command line.
  - Add a test to check that 'emconfigure cmake' method of using the toolchain file works.
  - Support the CMAKE_EXECUTABLE_SUFFIX method to specify the output for the build (.html/.js).