configs: Warn about pointer arithmetic.

It's not portable. Warn to help catching it early.
diff --git a/configs/linux b/configs/linux
index 73a6b61..c60f0d8 100644
--- a/configs/linux
+++ b/configs/linux
@@ -23,11 +23,11 @@
 X11_INCLUDES = -I/usr/X11R6/include
 
 CFLAGS = -Wall -Wmissing-prototypes -Wdeclaration-after-statement \
-	$(OPT_FLAGS) $(PIC_FLAGS) $(ARCH_FLAGS) \
-	$(DEFINES) $(ASM_FLAGS) $(X11_INCLUDES) -std=c99 -ffast-math 
+	-Wpointer-arith $(OPT_FLAGS) $(PIC_FLAGS) $(ARCH_FLAGS) \
+	$(DEFINES) $(ASM_FLAGS) $(X11_INCLUDES) -std=c99 -ffast-math
 
-CXXFLAGS = -Wall $(OPT_FLAGS) $(PIC_FLAGS) $(ARCH_FLAGS) $(DEFINES) \
-	$(X11_INCLUDES) 
+CXXFLAGS = -Wall -Wpointer-arith $(OPT_FLAGS) $(PIC_FLAGS) $(ARCH_FLAGS) \
+	$(DEFINES) $(X11_INCLUDES)
 
 # Work around aliasing bugs - developers should comment this out
 CFLAGS += -fno-strict-aliasing