Make CFLAGS more strict

This is a gcc-ism, but as we use gcc exclusively, we can use them.

Taken from one of Junio's mails. (Reminded to cherry-pick this patch
by one of Karsten Blees' mails.)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Makefile

@@ -308,7 +308,10 @@ endif
 
 # CFLAGS and LDFLAGS are for the users to override from the command line.
 
-CFLAGS = -g -O2 -Wall
+CFLAGS = -g -O2 -Wall -Werror \
+	-Wno-pointer-to-int-cast \
+	-Wold-style-definition \
+	-Wdeclaration-after-statement
 LDFLAGS =
 ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)