Use perl directly instead of grep -P

BSD grep (the one on OSX) lacks a -P flag. The grep -P command thus
fails and prints a usage message to the terminal. This fixes that.

tmk_core/rules.mk

@@ -371,7 +371,7 @@ show_path:
 	@echo OBJ=$(OBJ)
 
 check-size:
-	$(eval MAX_SIZE=$(shell n=`avr-gcc -E -mmcu=$(MCU) $(CFLAGS) $(OPT_DEFS) tmk_core/common/avr/bootloader_size.c 2> /dev/null | grep -oP "(?<=AVR_SIZE: ).+"`; echo $$(($$n)) || echo 0))
+	$(eval MAX_SIZE=$(shell n=`avr-gcc -E -mmcu=$(MCU) $(CFLAGS) $(OPT_DEFS) tmk_core/common/avr/bootloader_size.c 2> /dev/null | perl -ne 'print "$&\n" if /(?<=AVR_SIZE: ).+/'`; echo $$(($$n)) || echo 0))
 	$(eval CURRENT_SIZE=$(shell if [ -f $(BUILD_DIR)/$(TARGET).hex ]; then $(SIZE) --target=$(FORMAT) $(BUILD_DIR)/$(TARGET).hex | $(AWK) 'NR==2 {print $$4}'; else printf 0; fi))
 	if [ $(MAX_SIZE) -gt 0 ] && [ $(CURRENT_SIZE) -gt 0 ]; then \
 		$(SILENT) || printf "$(MSG_CHECK_FILESIZE)" | $(AWK_CMD); \