[feature] CMakeToolchain + Emscripten PoC

[memsharded]

Implement a proof of concept of using the new CMakeToolchain feature for building Emscripten/WebAsm projects:

• From Linux
• From Windows

Notes:

• Emscripten can be assumed installed
• Focused on creating/building 1 package, both local flow (conan install + native cmake) and conan create flow.
• Integration test.
• Test can be skipped for CI, but should work locally, annotate assumptions and installation details
• No env-var configuration at all
• All code must be private and local to the toolchains package
• New cross-build model with contexts must be used if necessary, not the old os_build, arch_build
• If enough time to implement reuse of a package, the cmake_find_package_multi generator must be used

[platc2]

Do I understand correctly this would include an example where I could use the following commands to build my project (Additional parameters are missing obviously)?

• conan install
• cmake
• cmake --build

I am looking for exactly this and I try to not include any conan macros in my CMakeLists.txt. I've got this working quite fine with cmake_find_package_multi and no Emscripten. Does PoC in this issue mean it would already be possible or are changes necessary for this?

[solvingj]

@platc2 yes, the current cmake_find_package generator now supports fully transparent (to the consuming CMakeLists.txt) delivery of Conan dependency and build variables to CMake. In some cases, it depends a bit on the packages themselves, and whether they've been refactored with some recent changes (using components feature), but the most crucial ones I am aware of are boost and openssl and they both have been.

So, here's example usage we used in a recent demo we did at C++ dublin meetup.

The key remaining caveat to note is for the caller, and that is we must still tell CMake to look in the "current directory" for .cmake files first.

Linux:

        conan install ..
        cmake ..  -DCMAKE_BUILD_TYPE=Release  -DCMAKE_MODULE_PATH=$PWD
        cmake --build .   

Windows:

        conan install ..
        cmake ..  -DCMAKE_BUILD_TYPE=Release  -DCMAKE_MODULE_PATH=%CD:\=/%
        cmake --build . --config Release

[memsharded]

Implemented in #18432, for next Conan 2.18 release, feedback very welcome!