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/run_gunzip.cxx

@@ -55,7 +55,7 @@ CopyGunzip(FILE *_dest, Path src_path)
 }
 
 int
-main(int argc, gcc_unused char **argv)
+main(int argc, [[maybe_unused]] char **argv)
 try {
 	if (argc != 2) {
 		fprintf(stderr, "Usage: run_gunzip PATH\n");