*: use gcc.h macros instead of GLib

src/db/ProxyDatabasePlugin.cxx

@@ -76,7 +76,7 @@ protected:
 	bool Configure(const config_param &param, GError **error_r);
 };
 
-G_GNUC_CONST
+gcc_pure
 static inline GQuark
 libmpdclient_quark(void)
 {
@@ -107,7 +107,7 @@ static constexpr struct {
 	{ TAG_NUM_OF_ITEM_TYPES, MPD_TAG_COUNT }
 };
 
-G_GNUC_CONST
+gcc_const
 static enum mpd_tag_type
 Convert(enum tag_type tag_type)
 {

src/db/SimpleDatabasePlugin.cxx

@@ -33,7 +33,7 @@
 #include <sys/types.h>
 #include <errno.h>
 
-G_GNUC_CONST
+gcc_const
 static inline GQuark
 simple_db_quark(void)
 {
@@ -241,7 +241,7 @@ SimpleDatabase::ReturnSong(gcc_unused Song *song) const
 #endif
 }
 
-G_GNUC_PURE
+gcc_pure
 const Directory *
 SimpleDatabase::LookupDirectory(const char *uri) const
 {