Check if cmake is on the PATH in emcmake.py

Currently, users who run emcmake without cmake on the PATH are greeted
with confusing errors like:

emcmake cmake ..
configure: cmake .. -DCMAKE_TOOLCHAIN_FILE=D:\src\emsdk\upstream\emscripten\cmake\Modules\Platform\Emscripten.cmake -DCMAKE_CROSSCOMPILING_EMULATOR=D:/src/emsdk/node/18.20.3_64bit/bin/node.exe -G Ninja
emcmake: error: 'cmake .. -DCMAKE_TOOLCHAIN_FILE=D:\src\emsdk\upstream\emscripten\cmake\Modules\Platform\Emscripten.cmake -DCMAKE_CROSSCOMPILING_EMULATOR=D:/src/emsdk/node/18.20.3_64bit/bin/node.exe -G Ninja' failed: [WinError 2] The system cannot find the file specified

Author: Peter0x44

emcmake.py

@@ -44,6 +44,10 @@ def has_substr(args, substr):
     # See https://github.com/emscripten-core/emscripten/issues/15522
     args.append(f'-DCMAKE_CROSSCOMPILING_EMULATOR={node_js}')
 
+  if not shutil.which('cmake'):
+    print('emcmake: cmake not found on PATH')
+    return 1
+
   # On Windows specify MinGW Makefiles or ninja if we have them and no other
   # toolchain was specified, to keep CMake from pulling in a native Visual
   # Studio, or Unix Makefiles.