Fix compilation issues on Debian Wheezy

.. which uses Arduino 1.0.1 with gcc-avr 4.7.2-2.

SIG_USART0_RECV is forbidden to use, even in a #ifdef test.

PROGMEM data must additionally be const. (not all instances fixed,
for instance I didn't touch he NUM_EXTRUDER>2 case)

prog_char is deprecated but available by defining __PROG_TYPES_COMPAT__
before including pgmspace.h.

This probably works on older Arduino IDEs and older GCC versions, but  I can't verify it.

Others have encountered similar things on other forks of Repetier, e.g., http://forums.reprap.org/read.php?267,185952,185952