configure: only try -Qunused-arguments for clang

other compilers don't need this option, but gcc 3 and perhaps others
accept it despite not understanding it, then print warnings about it
at build time.

omitting it when not needed will also help shorten the command lines.

configure

@@ -507,7 +507,7 @@ tryflag CFLAGS_AUTO -Werror=pointer-arith
 # parameter to stop printing warnings about LDFLAGS passed during
 # compiling stage and CFLAGS passed during linking stage.
 #
-tryflag CFLAGS_AUTO -Qunused-arguments
+test "$cc_family" = clang && tryflag CFLAGS_AUTO -Qunused-arguments
 
 if test "x$warnings" = xyes ; then
 tryflag CFLAGS_AUTO -Wall