Use std::locale() on OS X and std::locale(“”) elsewhere.
Drop support for building on OS X 10.5. It is too much hassle to
work around the limitations of the no longer maintained Xcode 3.1.4
for Leopard. Running a 10.6-built app on 10.5 works fine through
through the use of the 10.5 SDK.
std::locale() seems to be the generally safer way to specify the
default locale and works on Linux and OS X at least. Thanks to
dasmetzelmaennchen for the pointer.
Remove some obsolete SCons related svn:ignores.