1. Install the cpio utility using your distribution's standard package management system. All the major Linux distributions, as well as Cygwin, should have this utility.
2. Download the Xcode DMG from Apple's Developer Tools download page. It is free, but you will need to register with them. It is also huge (~924 MB).
3. Extract the Packages/MacOSX10.4.Universal.pkg directory from the Xcode DMG. Do not use the Linux kernel's HFS+ loopback mount: it does not support the format used by this DMG, and your files will be corrupted. One program that is known to work is PowerISO, although it isn't free.
4. Extract the pax archive inside the Mac OS X 10.4 SDK package.
gunzip -c ../MacOSX10.4.Universal.pkg/Contents/Archive.pax.gz | cpio -i
5. The SDK files should now live in MacOSX10.4-Universal-SDK/Developer/SDKs/MacOSX10.4u.sdk in the current directory. You'll pass the path to this directory to the configure script in the include directory. Feel free to get rid of the Xcode DMG and the package directory now.