Fix rtmidi-config.in (test uses = for string comparison, not ==)

rtmidi-config.in

@@ -8,11 +8,11 @@ LIBRARY="@LIBS@"
 CXXFLAGS="@CXXFLAGS@"
 CPPFLAGS="@CPPFLAGS@"
 
-if (test "x$1" == "x--libs") ; then
+if (test "x$1" = "x--libs") ; then
   echo "$LIBRARY"
-elif (test "x$1" == "x--cxxflags") ; then
+elif (test "x$1" = "x--cxxflags") ; then
   echo "$CXXFLAGS"
-elif (test "x$1" == "x--cppflags") ; then
+elif (test "x$1" = "x--cppflags") ; then
   echo "$CPPFLAGS"
 else
   echo "Unknown option: $1"