There is some confusion here, I think. As best as I can tell, the original problem reported in the Stackoverflow question and to MacPorts involved using the clang compiler, not llvm-gcc and the Decimal problem reported here was only ever a problem when using clang.  It's not an issue when using Apple's llvm-gcc compiler because the pre-#11149 configure test works properly with llvm-gcc.  That test looks for "-fwrapv" in the compiler help string:

$ llvm-gcc --version
i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ llvm-gcc -v --help 2>/dev/null | grep -- -fwrapv
  -fwrapv                     Assume signed arithmetic overflow wraps around
$ gcc-4.2 --version
i686-apple-darwin11-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5666) (dot 3)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc-4.2 -v --help 2>/dev/null | grep -- -fwrapv
  -fwrapv                     Assume signed arithmetic overflow wraps around
$ clang --version
Apple clang version 2.1 (tags/Apple/clang-163.7.1) (based on LLVM 3.0svn)
Target: x86_64-apple-darwin11.1.0
Thread model: posix
$ clang -v --help 2>/dev/null | grep -- -fwrapv

The above was using the 3 Apple-supplied compilers with the OS X 10.7 (Lion) version of Xcode 4.1 (Build version 4B110).  The 10.6 compiler versions should work the same way.

So this issue should be closed as a duplicate of Issue11149.  I'm setting it to pending and will close it if there are no objections.