pyexiv2: fix build failure

```brew install pyexiv2``` failed with messages:

~~~text
==> /usr/local/opt/scons/bin/scons BOOSTLIB=boost_python-mt
#include "exiv2/image.hpp"
         ^
1 error generated.
scons: *** [build/exiv2wrapper.os] Error 1
scons: building terminated because of errors.
~~~

This PR fixes the problem by appending additional ```CXXFLAGS``` options.