cmake - fix lrintf, lrint detection

This commit addresses the issues of not finding lrintf and lrint. We switch to check_symbol_exists instead per cmake documentation. Also make sure to link math lib for detection for nix.

For MSVC the issue for non x86 builds was that the standard was set to default which is 199409L. This resulted in not using lrintf even that it was found. To address this we set the C standard to C11 and it will only apply to newer versions of MSVC where the /std flag is supported.

Tested here: https://github.com/xnorpx/opus/runs/7208825734?check_suite_focus=true

Also tested manually on linux arm and windows arm64 (build)