gmacros: Add G_GNUC_FLAG_ENUM

This expands to __attribute__((flag_enum)) if available. It is useful when
using clang-tidy to do static analysis, which will warn about code like
(GParamFlags)(G_PARAM_CONSTRUCT_ONLY | G_PARAM_READWRITE).

glib/gmacros.h

@@ -779,6 +779,38 @@
 #define GLIB_CANNOT_IGNORE_DEPRECATIONS
 #endif
 
+/**
+ * G_GNUC_FLAG_ENUM:
+ *
+ * Expands to the GNU C `flag_enum` attribute if the compiler is gcc or clang.
+ * This attribute indicates that an enumerated type is used in bitwise
+ * operations.
+ * It is sometimes used in static analysis.
+ *
+ * See the
+ * [GNU C documentation](https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-flag_005fenum-type-attribute)
+ * for details.
+ *
+ * |[<!-- language="C" -->
+ * typedef enum {
+ *   G_KEY_FILE_NONE              = 0,
+ *   G_KEY_FILE_KEEP_COMMENTS     = 1 << 0,
+ *   G_KEY_FILE_KEEP_TRANSLATIONS = 1 << 1
+ * } G_GNUC_FLAG_ENUM GKeyFileFlags;
+ * ]|
+ *
+ * (The attribute can also be placed after `enum` and before the opening brace,
+ * but that may cause it to be misinterpreted as the name of the enum if the
+ * macro is not defined.)
+ *
+ * Since: 2.88
+ */
+#if g_macro__has_attribute(flag_enum)
+#define G_GNUC_FLAG_ENUM __attribute__((flag_enum))
+#else
+#define G_GNUC_FLAG_ENUM
+#endif
+
 /**
  * G_GNUC_MAY_ALIAS:
  *