Add OPT value in posix/Makefile to hold desired optimization

Useful when testing effect of various optimization levels to not
have to edit Makefile or reproduce CFLAGS on the command line

Signed-off-by: Keith Packard <keithp@keithp.com>

posix/Makefile

@@ -22,7 +22,9 @@ all: snek
 
 include $(SNEK_ROOT)/snek.defs
 
-CFLAGS=-DSNEK_MEM_INCLUDE_NAME -O0 -g -I. $(SNEK_CFLAGS)
+OPT=-O0
+
+CFLAGS=-DSNEK_MEM_INCLUDE_NAME $(OPT) -g -I. $(SNEK_CFLAGS)
 
 snek: $(SNEK_OBJ)
 	$(CC) $(CFLAGS) -o $@ $(SNEK_OBJ) -lm