Add a flag to disable appending Win32 port name identifier

Closes #100.

RtMidi.cpp

@@ -46,6 +46,10 @@
   #endif
 #endif
 
+// Default for Windows is to add an identifier to the port names; this
+// flag can be undefined to disable this behaviour.
+#define RTMIDI_ENSURE_UNIQUE_PORTNAMES
+
 //*********************************************************************//
 //  RtMidi Definitions
 //*********************************************************************//
@@ -2216,10 +2220,12 @@ std::string MidiInWinMM :: getPortName( unsigned int portNumber )
   // Next lines added to add the portNumber to the name so that 
   // the device's names are sure to be listed with individual names
   // even when they have the same brand name
+#ifdef RTMIDI_ENSURE_UNIQUE_PORTNAMES
   std::ostringstream os;
   os << " ";
   os << portNumber;
   stringName += os.str();
+#endif
 
   return stringName;
 }
@@ -2291,9 +2297,11 @@ std::string MidiOutWinMM :: getPortName( unsigned int portNumber )
   // the device's names are sure to be listed with individual names
   // even when they have the same brand name
   std::ostringstream os;
+#ifdef RTMIDI_ENSURE_UNIQUE_PORTNAMES
   os << " ";
   os << portNumber;
   stringName += os.str();
+#endif
 
   return stringName;
 }