remove gcc_unused

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

test/MakeTag.hxx

@@ -22,7 +22,7 @@
 #include "util/Compiler.h"
 
 inline void
-BuildTag(gcc_unused TagBuilder &tag) noexcept
+BuildTag([[maybe_unused]] TagBuilder &tag) noexcept
 {
 }