remove -O3

Index: setup.py
--- setup.py.orig
+++ setup.py
@@ -117,7 +117,6 @@ elif compiler in ('unix', 'mingw32'):
         extension_kwargs = pkgconfig_parse('liblz4')
     else:
         extension_kwargs['extra_compile_args'] = [
-            '-O3',
             '-Wall',
             '-Wundef'
         ]
