AP_Common: PSTR def has specific version for avr-gcc 4.6 compatibility

* Also a small change in FastSerial/vprintf.cpp, which duplicates the
  PSTR definition
* Won't effect others using earlier versions of avr-gcc

libraries/AP_Common/AP_Common.h

@@ -94,8 +94,14 @@ typedef struct {
  #endif
 
  # undef PSTR
+  /* Need const type for progmem - new for avr-gcc 4.6 */
+  # if __AVR__ && __GNUC__ == 4 && __GNUC_MINOR__ > 5 
+ # define PSTR(s) (__extension__({static const prog_char __c[] PROGMEM = (s); \
+                                  (const prog_char_t *)&__c[0]; }))
+  #else
  # define PSTR(s) (__extension__({static prog_char __c[] PROGMEM = (s); \
                                   (prog_char_t *)&__c[0]; }))
+  #endif
 #endif
 
 // a varient of PSTR() for progmem strings passed to %S in printf()

libraries/FastSerial/vprintf.cpp

@@ -52,7 +52,12 @@ extern "C" {
 #undef PROGMEM 
 #define PROGMEM __attribute__(( section(".progmem.data") )) 
 #undef PSTR 
+  /* Need const type for progmem - new for avr-gcc 4.6 */
+ #if __GNUC__ == 4 && __GNUC_MINOR__ > 5
+#define PSTR(s) (__extension__({static const prog_char __c[] PROGMEM = (s); &__c[0];})) 
+ #else
 #define PSTR(s) (__extension__({static prog_char __c[] PROGMEM = (s); &__c[0];})) 
+ #endif
 
 #ifdef GETBYTE
 #undef GETBYTE