clang: use gcc syntax to enable diagnostics color

clang has supported gcc syntax since version 3.3.0 from 10 years ago.
It's better than its own version because it takes a "when" verb which
allows us to explicitely ask for "auto".  This is useful when overriding
flags that came from elsewhere.

Before this patch, meson was just treating b_colorout="auto" as "always".

mesonbuild/compilers/mixins/clang.py

@@ -31,9 +31,9 @@
     from ...dependencies import Dependency  # noqa: F401
 
 clang_color_args: T.Dict[str, T.List[str]] = {
-    'auto': ['-fcolor-diagnostics'],
-    'always': ['-fcolor-diagnostics'],
-    'never': ['-fno-color-diagnostics'],
+    'auto': ['-fdiagnostics-color=auto'],
+    'always': ['-fdiagnostics-color=always'],
+    'never': ['-fdiagnostics-color=never'],
 }
 
 clang_optimization_args: T.Dict[str, T.List[str]] = {