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/test_vorbis_encoder.cxx

@@ -36,7 +36,7 @@
 static uint8_t zero[256];
 
 int
-main(gcc_unused int argc, gcc_unused char **argv)
+main([[maybe_unused]] int argc, [[maybe_unused]] char **argv)
 try {
 	/* create the encoder */