build-sys: Use common gcc argument detection for negative warning flags.

build-sys: Use common gcc argument detection for negative warning flags.

Starting with meson 0.46, it is able to detect these argument correctly.

See this commit in meson codebase for more details:
https://github.com/mesonbuild/meson/commit/695b8f3a0377d3e2ce78ba8716adc365b18edea1

We already carry a requirement for meson_version : '>= 0.46', so we can be sure
our build system will include this commit.

Tested by building systemd using a cloned meson synced to the 0.46.0 tag,
confirmed the warnings were detected correctly in that case. The meson messages included this snippet:

> Compiler for C supports arguments -Wno-unused-parameter -Wunused-parameter: YES
> Compiler for C supports arguments -Wno-missing-field-initializers -Wmissing-field-initializers: YES
> Compiler for C supports arguments -Wno-unused-result -Wunused-result: YES
> Compiler for C supports arguments -Wno-format-signedness -Wformat-signedness: YES
> Compiler for C supports arguments -Wno-error=nonnull -Werror=nonnull: YES
> Compiler for C supports arguments -Wno-maybe-uninitialized -Wmaybe-uninitialized: YES