make code compatible with up-to-date build environment

These changes make it possible to compile open9x with old (avr-gcc 4.3.3, 
avr-libc 1.6.7) and up-to-date (avr-gcc 4.6.2, avr-libc 1.8.0) build
environments. See Issue 1 for more details.

src/beeper.cpp

@@ -32,7 +32,7 @@ uint8_t hapticTick = 0;
 #endif
 
 // The various "beep" tone lengths
-static prog_uint8_t APM beepTab[]= {
+static const pm_uint8_t beepTab[] PROGMEM = {
     // 0   1   2   3    4
     0,  1,  8, 30, 100, //silent
     1,  1,  8, 30, 100, //xShort