Use -O3 instead of -Ofast

Because we use NAN in a few places in the code at the moment, so
using -ffast-math could be an issue, and new release of clang
complains about it.

Would still like to avoid using NAN or INFINITY in places where
we don't really have to.

SConstruct

@@ -73,7 +73,7 @@ if env['werror']:
     env.Append(CCFLAGS='-Werror')
 
 if env['mode'] not in ['debug', 'analyze']:
-    env.Append(CPPDEFINES='NDEBUG', CCFLAGS='-Ofast')
+    env.Append(CPPDEFINES='NDEBUG', CCFLAGS='-O3')
 
 if env['mode'] == 'debug':
     env.Append(CCFLAGS=['-O0'])