Fix CMAKE_FIND_ROOT_PATH

This allows projects and libraries using CMake to automatically find include directories and library files in the emscripten system directory. I am unsure why it was set to `${EMSCRIPTEN_ROOT_PATH}/cmake` before, but I tested it and it didn't work until I changed `cmake` to `system`.
`CMAKE_FIND_ROOT_PATH` is documented [here](http://www.cmake.org/Wiki/CMake_Cross_Compiling#Searching_and_finding_external_software).