Further work on WebAssembly target for Guix

[zamfofex]

Hello, Guix! For anyone who might have had interest in it last time, I have 
continued working on my WebAssembly cross‐compilation target for Guix, now with 
support for the Meson and CMake build systems and also preliminary support for 
C++.

Once again, it’s in fairly early stages currently, but it is able to compile 
fmtlib and GMP. My hope was to compile daikichi, but it seems it uses various 
constructs that are not supported yet (e.g. ‘#include <filesystem>’ is not 
supported).

See my changes on https://git.sr.ht/~zamfofex/guix-wasm To try it out, run the 
following command. (Though note that it will compile Clang from source!)

      ./pre-inst-env guix build --target=wasm32-unknown-wasi fmt-wasm gmp-wasm

Currently, there are WebAssembly‐specific packages and build systems for fmtlib 
and GMP in order to avoid a world rebuild.

Please do let me know what you think about it! I’m open for suggestions and 
requests, if anyone feels like this is an interesting endeavor.

This is a follow‐up to 
https://lists.gnu.org/archive/html/guix-devel/2023-09/msg00458.html