This is rather annoying, gcc doesn't accept multipe -isysroot flags on 
10.4, yet we need to specify -isysroot during configure to ensure that 
tests are done using the right SDK, otherwise most of configure will use 
the system headers instead of the SDK specified using the --enable-
universalsdk option.

I'm currently trying to teach configure to only add -isysroot to the 
configure-time CFLAGS when the OS version is 10.5 or later, because 
that's the only platform where -isysroot is really needed in the 
configure-time CFLAGS.

The patch should be ready later tonight, I'll post it here instead of 
committing to avoid interfering with the release process.