Makefile: Warn on overflow undefined behavior

To ensure our sanity checks stay sane, it's good to make sure we aren't
invoking undefined overflow behavior. Make the compiler warn us when it
attempts to take advantage of undefined overflow in optimizing.

Author: Patater

Makefile

@@ -137,7 +137,7 @@ endif
 
 SYSLIBS:=-lgcc -lc -lnosys
 DEBUG:=-g3
-WARNING:=-Wall -Werror
+WARNING:=-Wall -Wstrict-overflow=3 -Werror
 
 # Select optimizations depending on the build mode.
 ifeq ("$(BUILD_MODE)","debug")