use [[gnu::...]] attributes

src/db/plugins/simple/Directory.cxx

@@ -193,7 +193,7 @@ Directory::FindSong(std::string_view name_utf8) const noexcept
 	return nullptr;
 }
 
-gcc_pure
+[[gnu::pure]]
 static bool
 directory_cmp(const Directory &a, const Directory &b) noexcept
 {

src/db/plugins/simple/DirectorySave.cxx

@@ -25,7 +25,7 @@
 #define DIRECTORY_BEGIN "begin: "
 #define DIRECTORY_END "end: "
 
-gcc_const
+[[gnu::const]]
 static const char *
 DeviceToTypeString(unsigned device) noexcept
 {
@@ -44,7 +44,7 @@ DeviceToTypeString(unsigned device) noexcept
 	}
 }
 
-gcc_pure
+[[gnu::pure]]
 static unsigned
 ParseTypeString(const char *type) noexcept
 {

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -248,7 +248,7 @@ SimpleDatabase::ReturnSong([[maybe_unused]] const LightSong *song) const noexcep
 	}
 }
 
-gcc_const
+[[gnu::const]]
 static DatabaseSelection
 CheckSelection(DatabaseSelection selection) noexcept
 {

src/db/plugins/simple/Song.cxx

@@ -47,7 +47,7 @@ Song::GetURI() const noexcept
 /**
  * Path name traversal of a #Directory.
  */
-gcc_pure
+[[gnu::pure]]
 static const Directory *
 FindTargetDirectory(const Directory &base, std::string_view path) noexcept
 {
@@ -69,7 +69,7 @@ FindTargetDirectory(const Directory &base, std::string_view path) noexcept
 /**
  * Path name traversal of a #Song.
  */
-gcc_pure
+[[gnu::pure]]
 static const Song *
 FindTargetSong(const Directory &_directory, std::string_view target) noexcept
 {

src/db/plugins/simple/SongSort.cxx

@@ -60,7 +60,7 @@ compare_tag_item(const Tag &a, const Tag &b, TagType type) noexcept
 }
 
 /* Only used for sorting/searchin a songvec, not general purpose compares */
-gcc_pure
+[[gnu::pure]]
 static bool
 song_cmp(const Song &a, const Song &b) noexcept
 {