Problem building emscripten cross

[j-mckim]

Attempting to run cmake to build the webasm environment fails and elicits these two diagnostics:

$ mkdir b3 && cd b3 && emcmake cmake ..
configure: cmake .. -DCMAKE_TOOLCHAIN_FILE=/home/jim/src/emsdk/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake -DCMAKE_CROSSCOMPILING_EMULATOR=/home/jim/src/emsdk/node/14.15.5_64bit/bin/node;--experimental-wasm-threads
-- NanoGUI v0.1.4
-- Setting build type to 'Release' as none was specified.
-- NanoGUI: using GLES 2 backend.
-- NanoGUI: building static library.
CMake Error at CMakeLists.txt:505 (install):
install DIRECTORY given no DESTINATION!

CMake Error at CMakeLists.txt:525 (configure_package_config_file):
Unknown CMake command "configure_package_config_file".

-- Configuring incomplete, errors occurred!
See also "/home/jim/src/nanogui/b3/CMakeFiles/CMakeOutput.log".
emcmake: error: 'cmake .. -DCMAKE_TOOLCHAIN_FILE=/home/jim/src/emsdk/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake -DCMAKE_CROSSCOMPILING_EMULATOR=/home/jim/src/emsdk/node/14.15.5_64bit/bin/node;--experimental-wasm-threads' failed (returned 1)

This is using the current emscripten, via a git clone.

Per some advice I read while researching the diagnostics, I patched CMakeLists.txt to 'include(CMakePackageConfigHelpers)' patch.txt
. That eliminated the "unknown command" diagnostic.

With respect to the "install DIRECTORY given no DESTINATION!" diagnostic, I'm working around that by making NANOGUI_INSTALL default to 0. I know this isn't the correct fix, but I don't have a good understanding of what is the correct action.