doc: explicitly mention CMAKE_PREFIX_PATH in building docs.
doc: explicitly mention CMAKE_PREFIX_PATH in building docs.

I ran into a weird problem with Corrade module path not being found when
using just CMAKE_INSTALL_PREFIX for Emscripten build. Amazingly enough,
this wasn't problem at all with all other crosscompilation builds. CMake
hates me, I think.

MiUishadow/magnum,ashimidashajia/magnum,DerThorsten/magnum,DerThorsten/magnum,MiUishadow/magnum,MiUishadow/magnum,DerThorsten/magnum,ashimidashajia/magnum,DerThorsten/magnum,MiUishadow/magnum,MiUishadow/magnum,ashimidashajia/magnum,ashimidashajia/magnum,DerThorsten/magnum,ashimidashajia/magnum,MiUishadow/magnum