docs: Document usage with Xcode 4.1

With Xcode 4.1 'gcc' is symlinked llvm-gcc-4.2 which cannot bootstrap
other gccs.
See http://llvm.org/bugs/show_bug.cgi?id=9571

On my machine the faulty gcc is
gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)

Document a workaround.

Signed-off-by: "Titus von Boxberg" <titus@v9g.de>

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }