We just went through this with another thread, but here goes:
To build for the web, you have to specify the correct path for your Emscripten installation. Download and install Emscripten as described at https://emscripten.org/docs/getting_started/downloads.html Once you have downloaded, installed and activated the latest, you should have a folder called emsdk at the location you cloned the repo to. When you invoke the cmake_emscripten.sh bash script located in Urho3D/script, you have to specify the proper path inside the emsdk installation in order for CMake to find the emscripten build tools. If this path is not properly specified, you get the error you have, of CMake being unable to find the build tools.
So, say you have installed emsdk into /home/codefive/emsdk, and you have cloned Urho3D repo into /home/codefive/Urho3D. You could configure the project by changing dir to Urho3D and calling:
./script/cmake_emscripten.sh /home/codefive/Urho3D_WebBuild -DEMSCRIPTEN_ROOT_PATH=/home/codefive/emsdk/upstream/emscripten
This will configure for a web build. Specify any other options as you desire (ie, to turn on/off Lua and AngelScript, etc…). Upon completion, navigate to /home/codefive/Urho3D_WebBuild and invoke make. It should build the Urho3D libraries.
Note that as we established in the other thread, you have to bump the required version of CMake as specified in the CMakeLists.txt files, in order to use the latest Emscripten release. I believe you can go back to 1.38.4 version of Emscripten if you don’t want to edit the CMakeLists files, though CodeBlocks should make it easy to change all of them by using the Replace In Files, and replacing 3.2.3 with the desired CMake min version. I have established that 3.14.5 works acceptably. If you do revert to Emscripten version 1.38.4, then the path you use for EMSCRIPTEN_ROOT_PATH will be /home/codefive/emsdk/fastcomp/emscripten instead of /home/codefive/emsdk/upstream/emscripten. I recommend just sticking with Emscripten latest and updating the CMake files, personally.
Note that you might not be able to build a project for CodeBlocks when doing a web build. I didn’t try very hard, but every time I did try it wanted to build a native build instead. You might have to do some fiddling around to get that to work. Look at the bash scripts in /script for cmake_generic, cmake_codeblocks and cmake_emscripten to see what they do and whether or not they can be made to do what you want.
Also note that when using these build scripts, you don’t need to invoke rake. You can use rake to set up a new project that uses Urho3D if you desire, or you can just build a simple CMakeLists.txt by hand, and use the same script/cmake_emscripten.sh that the library uses in order to configure it.