Compile with -Werror (treat warnings as errors)

src/Makefile

@@ -36,7 +36,7 @@ CC= $(DEFAULT_CC)
 # to slow down the C part by not omitting it. Debugging, tracebacks and
 # unwinding are not affected -- the assembler part has frame unwind
 # information and GCC emits it where needed (x64) or with -g (see CCDEBUG).
-CCOPT= -O2 -fomit-frame-pointer
+CCOPT= -O2 -fomit-frame-pointer -Werror
 # Use this if you want to generate a smaller binary (but it's slower):
 #CCOPT= -Os -fomit-frame-pointer
 # Note: it's no longer recommended to use -O3 with GCC 4.x.