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/software_volume.cxx

@@ -58,7 +58,7 @@ try {
 
 	while ((nbytes = read(0, buffer, sizeof(buffer))) > 0) {
 		auto dest = pv.Apply({buffer, size_t(nbytes)});
-		gcc_unused ssize_t ignored = write(1, dest.data, dest.size);
+		[[maybe_unused]] ssize_t ignored = write(1, dest.data, dest.size);
 	}
 
 	pv.Close();