Share top level Makefile between emscripten, linux and nacl.

Remove the custom top level Makefile.emscripten and instead
add build-emscripten.sh which can be used like build-linux.sh
and build-nacl.sh.

Also, use the top level lib folder to store emscripten libs.
This brings it in line with the other make-based builds.
Fix emscripten build on linux

Remove the dependacy of the emscripten build on /Library/Fonts
(a darwin-only folder).  On non-darwin platforms we use the
ariel.tff that is part of TestCpp.
Merge pull request #2998 from sbc100/emscripten_linux

closed #2333: 1) Share top level Makefile between emscripten, linux and nacl.
2) Remove the dependacy of the emscripten build on /Library/Fonts
(a darwin-only folder).  On non-darwin platforms we use the
ariel.tff that is part of TestCpp.