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:
695b8f3a03

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

meson.build

@@ -321,6 +321,20 @@ possible_cc_flags = [
         '-Werror=shift-overflow=2',
         '-Wdate-time',
         '-Wnested-externs',
+
+        # negative arguments are correctly detected starting with meson 0.46.
+        '-Wno-unused-parameter',
+        '-Wno-missing-field-initializers',
+        '-Wno-unused-result',
+        '-Wno-format-signedness',
+
+        # work-around for gcc 7.1 turning this on on its own.
+        '-Wno-error=nonnull',
+
+        # Disable -Wmaybe-uninitialized, since it's noisy on gcc 8 with
+        # optimizations enabled, producing essentially false positives.
+        '-Wno-maybe-uninitialized',
+
         '-ffast-math',
         '-fno-common',
         '-fdiagnostics-show-option',
@@ -364,23 +378,6 @@ endif
 add_project_arguments(cc.get_supported_arguments(possible_cc_flags), language : 'c')
 add_project_link_arguments(cc.get_supported_link_arguments(possible_link_flags), language : 'c')
 
-# "negative" arguments: gcc on purpose does not return an error for "-Wno-"
-# arguments, just emits a warning. So test for the "positive" version instead.
-foreach arg : ['unused-parameter',
-               'missing-field-initializers',
-               'unused-result',
-               'format-signedness',
-               'error=nonnull', # work-around for gcc 7.1 turning this on on its own
-
-               # Disable -Wmaybe-uninitialized, since it's noisy on gcc 8 with
-               # optimizations enabled, producing essentially false positives.
-               'maybe-uninitialized',
-              ]
-        if cc.has_argument('-W' + arg)
-                add_project_arguments('-Wno-' + arg, language : 'c')
-        endif
-endforeach
-
 if cc.compiles('''
    #include <time.h>
    #include <inttypes.h>