Compiling a 10.4u Application with GCC 4.2 on OS X

It turns out you can build an OS X 10.4 compatible application using GCC 4.2.  The trick is: you need to install GCC 4.2 Developer Preview 1, available as a download from Apple via ADC.

The developer preview has the GCC 4.2 support files for Darwin 9 (that is, OS X 10.5) in the 10.4u SDK.