using std::string's += operator instead of (more efficient) append for STLPORT + MSVC <= 7.0

msvc-6-stlport fails to find basic_string::append( Iter, Iter) when linking
with Iter = _STL::_DBG_iter< _STL::_Nondebug_string<char,
                                                    _STL::char_traits<char>,
                                                    _STL::allocator<char> >
might affect other MSwindows compilers.


[SVN r22235]

libs/format

@@ -1 +1 @@
-Subproject commit cf14ff1f05ca1717a85fbeec6c0b978834951083
+Subproject commit 14b13e2458abf1030ee600f990547473a6aa7d24