meson: detect availability of attributes using cc.has_function_attribute()

Alternative to fabc22f599

As suggested in https://github.com/systemd/systemd/pull/41174#discussion_r2966411375

src/boot/meson.build

@@ -80,10 +80,13 @@ endif
 efi_conf = configuration_data()
 
 # import several configs from userspace
-foreach name : ['HAVE_ATTRIBUTE_RETAIN',
-                'HAVE_WARNING_ZERO_AS_NULL_POINTER_CONSTANT',
-                'HAVE_WARNING_ZERO_LENGTH_BOUNDS',
-               ]
+foreach name : ['HAVE_WARNING_ZERO_LENGTH_BOUNDS',
+                'HAVE_WARNING_ZERO_AS_NULL_POINTER_CONSTANT']
+        efi_conf.set(name, conf.get(name))
+endforeach
+
+foreach attr : possible_c_attributes
+        name = 'HAVE_ATTRIBUTE_' + attr.to_upper()
         efi_conf.set(name, conf.get(name))
 endforeach
 

src/fundamental/macro-fundamental.h

@@ -98,24 +98,24 @@
 #define _weak_ __attribute__((__weak__))
 #define _weakref_(x) __attribute__((__weakref__(#x)))
 
+#if HAVE_ATTRIBUTE_ALLOC_SIZE
+#  define _alloc_(...) __attribute__((__alloc_size__(__VA_ARGS__)))
+#else
+#  define _alloc_(...)
+#endif
+
+#if HAVE_ATTRIBUTE_FALLTHROUGH
+#  define _fallthrough_ __attribute__((__fallthrough__))
+#else
+#  define _fallthrough_
+#endif
+
 #if HAVE_ATTRIBUTE_RETAIN
 #  define _retain_ __attribute__((__retain__))
 #else
 #  define _retain_
 #endif
 
-#ifdef __clang__
-#  define _alloc_(...)
-#else
-#  define _alloc_(...) __attribute__((__alloc_size__(__VA_ARGS__)))
-#endif
-
-#if defined(__clang__) && __clang_major__ < 10
-#  define _fallthrough_
-#else
-#  define _fallthrough_ __attribute__((__fallthrough__))
-#endif
-
 #if __GNUC__ >= 15
 #  define _nonnull_if_nonzero_(p, n) __attribute__((nonnull_if_nonzero(p, n)))
 #else