more [[gnu::...]] attributes

src/db/plugins/upnp/Directory.cxx

@@ -33,7 +33,7 @@
 /* this destructor exists here just so it won't get inlined */
 UPnPDirContent::~UPnPDirContent() = default;
 
-gcc_pure
+[[gnu::pure]]
 static UPnPDirObject::ItemClass
 ParseItemClass(StringView name) noexcept
 {
@@ -45,7 +45,7 @@ ParseItemClass(StringView name) noexcept
 		return UPnPDirObject::ItemClass::UNKNOWN;
 }
 
-gcc_pure
+[[gnu::pure]]
 static SignedSongTime
 ParseDuration(const char *duration) noexcept
 {
@@ -73,7 +73,7 @@ ParseDuration(const char *duration) noexcept
  * elements. There is a very slight risk of collision in doing
  * this. Twonky returns directory names (titles) like 'Artist/Album'.
  */
-gcc_pure
+[[gnu::pure]]
 static std::string &&
 TitleToPathSegment(std::string &&s) noexcept
 {

src/db/plugins/upnp/Directory.hxx

@@ -21,7 +21,6 @@
 #define MPD_UPNP_DIRECTORY_HXX
 
 #include "Object.hxx"
-#include "util/Compiler.h"
 
 #include <string>
 #include <vector>
@@ -39,7 +38,7 @@ public:
 
 	~UPnPDirContent();
 
-	gcc_pure
+	[[gnu::pure]]
 	UPnPDirObject *FindObject(std::string_view name) noexcept {
 		for (auto &o : objects)
 			if (o.name == name)

src/db/plugins/upnp/Object.hxx

@@ -21,7 +21,6 @@
 #define MPD_UPNP_OBJECT_HXX
 
 #include "tag/Tag.hxx"
-#include "util/Compiler.h"
 
 #include <string>
 
@@ -89,12 +88,12 @@ public:
 		tag.Clear();
 	}
 
-	gcc_pure
+	[[gnu::pure]]
 	bool IsRoot() const noexcept {
 		return type == Type::CONTAINER && id == "0";
 	}
 
-	gcc_pure
+	[[gnu::pure]]
 	bool Check() const noexcept {
 		return !id.empty() &&
 			/* root nodes don't need a parent id and a