doc: more Emscripten troubleshooting.

doc/platforms-html5.dox

@@ -173,7 +173,7 @@ cmake -DCMAKE_INSTALL_PREFIX=/path/to/my/emscripten/deploy ..
 cmake --build . --target install
 
 cd /path/to/my/emscripten/deploy
-python -m http.server
+python -m http.server # or python -m SimpleHTTPServer with Python 2
 @endcode
 
 After that, you can open http://localhost:8000 to see the files. Stop the
@@ -302,6 +302,23 @@ WebGL specification provide even more detail:
 
 @section platforms-html5-troubleshooting Troubleshooting
 
+@subsection platforms-html5-troubleshooting-macos Setting up Emscripten on macOS
+
+Emscripten is available through Homebrew:
+
+@code{.sh}
+brew install emscripten
+@endcode
+
+Because LLVM is also distributed as part of Xcode, it will get picked up by
+default. Emscripten however needs to use its own bundled version, so you may
+need to export the @cb{.sh} $LLVM @ce environment variable to port to the other
+location:
+
+@code{.sh}
+export LLVM=/usr/local/opt/emscripten/libexec/llvm/bin
+@endcode
+
 @subsection platforms-html5-troubleshooting-bootstrap First Emscripten run takes long or fails
 
 Emscripten downloads and builds a lot of things on first startup or after
@@ -332,6 +349,9 @@ echo "int main() {}" > main.cpp
 em++ -s WASM=1 main.cpp
 @endcode
 
+After this step it might be also needed to purge all CMake build directories
+and set them up again to ensure no bad state got cached.
+
 @subsection platforms-html5-troubleshooting-corrade-not-found CMake can't find _CORRADE_MODULE_DIR
 
 If initial CMake configuration fails with
@@ -341,13 +361,15 @@ If initial CMake configuration fails with
 @endcode
 
 The solution is to explicitly pass `CMAKE_PREFIX_PATH` pointing to directory
-where Corrade is installed, for example:
+where Corrade is installed. In some cases it might also be needed to point
+`CMAKE_FIND_ROOT_PATH` to the same location. For example:
 
 @code{.sh}
 mkdir build-emscripten && cd build-emscripte
 cmake .. \
     -DCMAKE_TOOLCHAIN_FILE=../toolchains/generic/Emscripten-wasm.cmake \
-    -DCMAKE_PREFIX_PATH=/usr/lib/emscripten/system/ \
+    -DCMAKE_PREFIX_PATH=/your/emscripten/libs/ \
+    -DCMAKE_FIND_ROOT_PATH=/your/emscripten/libs/ \
     -G Ninja
 @endcode