I think build-osx.txt needs to be modified slightly:

../configure --prefix="$PREFIX"
--with-osx_cocoa
--disable-shared
--disable-debug_flag
--with-macosx-version-min=10.5
--enable-stl
--enable-utf8
--enable-universal_binary
--with-libjpeg=builtin
--with-libpng=builtin
--with-regex=builtin
--with-libtiff=builtin
--with-zlib=builtin
--with-expat=builtin
--with-macosx-sdk=/Developer/SDKs/MacOSX10.5.sdk &&

Anyone trying to compile has to note the 10.5 sdk config flags - they need to be set to 10.6 as they are in earlier part of the instructions, otherwise it fails out like this:

<Value, is_const, cache>::hashtable_iterator(Internal::hash_node<Value, cache>) [with Value = std::pair<wxAnyValueType* const, wxVariantData* ()(const wxAny&)>, bool is_const = false, bool cache = false]
/Developer/SDKs/MacOSX10.5.sdk/usr/include/c++/4.2.1/tr1/hashtable:232: note: Internal::hashtable_iterator<Value, is_const, cache>::hashtable_iterator(Internal::hash_node<Value, cache>, Internal::hash_node<Value, cache>) [with Value = std::pair<wxAnyValueType* const, wxVariantData* ()(const wxAny&)>, bool is_const = false, bool cache = false]
/Developer/SDKs/MacOSX10.5.sdk/usr/include/c++/4.2.1/tr1/hashtable:225: note: Internal::hashtable_iterator<std::pair<wxAnyValueType const, wxVariantData* ()(const wxAny&)>, false, false>::hashtable_iterator(const Internal::hashtable_iterator<std::pair<wxAnyValueType const, wxVariantData* (*)(const wxAny&)>, false, false>&)
lipo: can't figure out the architecture type of: /var/folders/0d/0d0nJ9M9FLGZIyqsQjsi2U+++TI/-Tmp-//ccO0mcFc.out
make: *** [baselib_any.o] Error 1