cmake: prefer `COMPILE_OPTIONS` over `CMAKE_C_FLAGS` for custom C options

cmake: prefer `COMPILE_OPTIONS` over `CMAKE_C_FLAGS` for custom C options

Also:
- pass `-D_GNU_SOURCE` via `COMPILE_DEFINITIONS`.
- make it explicit to pass these C flags to feature checks.
- update `_GNU_SOURCE` comment with `pipe2()`.
- enable `-pedantic-errors` picky option for GCC with CMake <3.23.
- drop redundant condition when stripping existing MSVC `/Wn` options.

CMake passes `CMAKE_C_FLAGS` to targets, feature checks and raw
`try_compile()` calls. With `COMPILE_OPTIONS`, this is limited to
targets, and we must explicitly pass them to feature checks. This
makes the build logic clearer, and offers more control. It also
reduces log noise by omitting these options from linker commands,
and from `CMAKE_C_FLAGS` dumps in feature checks.

Closes #17047