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

src/command/AllCommands.cxx

@@ -204,7 +204,7 @@ static constexpr struct command commands[] = {
 
 static constexpr unsigned num_commands = std::size(commands);
 
-gcc_pure
+[[gnu::pure]]
 static bool
 command_available([[maybe_unused]] const Partition &partition,
 		  [[maybe_unused]] const struct command *cmd) noexcept
@@ -290,7 +290,7 @@ command_init() noexcept
 #endif
 }
 
-gcc_pure
+[[gnu::pure]]
 static const struct command *
 command_lookup(const char *name) noexcept
 {

src/command/FileCommands.cxx

@@ -34,14 +34,14 @@
 #include <cassert>
 #include <array>
 
-gcc_pure
+[[gnu::pure]]
 static bool
 SkipNameFS(PathTraitsFS::const_pointer name_fs) noexcept
 {
 	return PathTraitsFS::IsSpecialFilename(name_fs);
 }
 
-gcc_pure
+[[gnu::pure]]
 static bool
 skip_path(Path name_fs) noexcept
 {
@@ -87,7 +87,7 @@ handle_listfiles_local(Response &r, Path path_fs)
 #pragma GCC diagnostic pop
 #endif
 
-gcc_pure
+[[gnu::pure]]
 static bool
 IsValidName(const std::string_view s) noexcept
 {
@@ -99,7 +99,7 @@ IsValidName(const std::string_view s) noexcept
 	});
 }
 
-gcc_pure
+[[gnu::pure]]
 static bool
 IsValidValue(const std::string_view s) noexcept
 {

src/command/FingerprintCommands.cxx

@@ -101,7 +101,7 @@ GetChromaprintCommand::DecodeStream(InputStream &input_stream,
 	plugin.StreamDecode(*this, input_stream);
 }
 
-gcc_pure
+[[gnu::pure]]
 static bool
 decoder_check_plugin_mime(const DecoderPlugin &plugin,
 			  const InputStream &is) noexcept
@@ -113,7 +113,7 @@ decoder_check_plugin_mime(const DecoderPlugin &plugin,
 		plugin.SupportsMimeType(GetMimeTypeBase(mime_type));
 }
 
-gcc_pure
+[[gnu::pure]]
 static bool
 decoder_check_plugin_suffix(const DecoderPlugin &plugin,
 			    std::string_view suffix) noexcept
@@ -123,7 +123,7 @@ decoder_check_plugin_suffix(const DecoderPlugin &plugin,
 	return !suffix.empty() && plugin.SupportsSuffix(suffix);
 }
 
-gcc_pure
+[[gnu::pure]]
 static bool
 decoder_check_plugin(const DecoderPlugin &plugin, const InputStream &is,
 		     std::string_view suffix) noexcept

src/command/OutputCommands.cxx

@@ -71,7 +71,7 @@ IsValidAttributeNameChar(char ch) noexcept
 	return IsAlphaNumericASCII(ch) || ch == '_';
 }
 
-gcc_pure
+[[gnu::pure]]
 static bool
 IsValidAttributeName(const char *s) noexcept
 {

src/command/PartitionCommands.cxx

@@ -44,7 +44,7 @@ IsValidPartitionChar(char ch)
 	return IsAlphaNumericASCII(ch) || ch == '-' || ch == '_';
 }
 
-gcc_pure
+[[gnu::pure]]
 static bool
 IsValidPartitionName(const char *name) noexcept
 {
@@ -56,7 +56,7 @@ IsValidPartitionName(const char *name) noexcept
 	return true;
 }
 
-gcc_pure
+[[gnu::pure]]
 static bool
 HasPartitionNamed(Instance &instance, const char *name) noexcept
 {

src/command/StorageCommands.cxx

@@ -22,7 +22,7 @@
 
 #include <memory>
 
-gcc_pure
+[[gnu::pure]]
 static bool
 skip_path(const char *name_utf8) noexcept
 {