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>

# Default test suite configuration

# Default DejaGnu configuration

DG_TOOLNAME = gcc

DG_TARGET_HOSTNAME = 127.0.0.1

DG_TARGET_USERNAME = root

# Default tests

DG_C_TESTS = 

DG_CPP_TESTS =