Workaround Clang -Wnontrivial-memcall, clean up some unnecessary GCC -Wclass-memaccess pragmas

[akien-mga]

The CMake config silences -Wclass-memaccess globally, but some of the warnings have been resolved recently with 100ace8 for Emscripten. Clang is also getting similar warnings in its latest versions, sadly they picked a different name than GCC (-Wnontrivial-memcall), so the diagnostic ignore pragmas need to exclude all Clang-derived compilers, not just Emscripten.

Since you had started working around the warnings with (void *) casts I applied the same approach where I could, but please review thoroughly as I didn't check whether this was fully equivalent to previous code / what the intention is.

Also added a quick GCC warning fix for -Wunused-local-typedefs.

Tested compiling on Fedora 42 with CMake 3.31.6 on both Release and Debug builds, with:

• GCC 15.2.1
• Clang 20.1.8
• Emscripten 4.0.10

Didn't test the library itself for potential behavior change.