Improve build: add <unistd.h> and use -O2 instead of -O3

(Nobody should use -O3. It just makes bad inlining decisions).

Makefile

@@ -7,7 +7,7 @@
 # BREAK YOUR LOCAL DIFFS! show-diff and anything using it will likely randomly
 # break unless your underlying filesystem supports those sub-second times
 # (my ext3 doesn't).
-CFLAGS=-g -O3 -Wall
+CFLAGS=-g -O2 -Wall
 
 CC=gcc
 AR=ar

cache.h

@@ -1,6 +1,7 @@
 #ifndef CACHE_H
 #define CACHE_H
 
+#include <unistd.h>
 #include <stdio.h>
 #include <sys/stat.h>
 #include <fcntl.h>