Remove hardcoded optimization level

Index: src/common.h
--- src/common.h.orig
+++ src/common.h
@@ -96,7 +96,7 @@ typedef unsigned long ulong;
 # define ATTR_UNUSED __attribute__((unused))
 # define ATTR_NORETURN __attribute__((noreturn))
 # define ATTR_PRINTFLIKE(fmt,var) __attribute__((format(printf,fmt,var)))
-# define ATTR_OPTIMIZE __attribute__((optimize("2")))
+# define ATTR_OPTIMIZE
 #else
 # define ATTR_UNUSED
 # define ATTR_NORETURN
