Modified the change to proton-c/CMakeLists.txt to use a more idiomatic test as suggested by Andrew Stitcher using a FindEmscripten.cmake module

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/branches/fadams-javascript-binding@1554617 13f79535-47bb-0310-9956-ffa450edef68