I'm not sure where the emscripten-$version came from, maybe that's
how it used to be? In any case, in the current version (3.1.x),
the installation folder is in upstream/emscripten, so add PATH
there to access all emscripten utils.
Pick-to: 6.4
Change-Id: I648feae21bc5caecc3f0bf0a7d16238680a85590
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>