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).

AUTHORS

@@ -146,3 +146,4 @@ a license to everyone to use it as detailed in LICENSE.)
 * Nicolas Guillemot <nlguillemot@gmail.com>
 * Sathyanarayanan Gunasekaran <gsathya.ceg@gmail.com> (copyright owned by Mozilla Foundation)
 * Nikolay Vorobyov <nik.vorobyov@gmail.com>
+* Jonas Platte <mail@jonasplatte.de>

cmake/Platform/Emscripten.cmake

@@ -49,7 +49,7 @@ if (NOT CMAKE_MODULE_PATH)
 endif()
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${EMSCRIPTEN_ROOT_PATH}/cmake/Modules")
 
-set(CMAKE_FIND_ROOT_PATH "${EMSCRIPTEN_ROOT_PATH}/cmake")
+set(CMAKE_FIND_ROOT_PATH "${EMSCRIPTEN_ROOT_PATH}/system")
 
 if (CMAKE_HOST_WIN32)
 	set(EMCC_SUFFIX ".bat")