Fix for bug #68114 (Build fails on OS X due to undefined symbols)

gcc (i686-apple-darwin10-gcc-4.2.1) on OS X cannot link fixed-width
decimals and fails with undefined symbols errors like ___extendsddf.
If configure used gcc for compiling it would notice and mark the
feature HAVE_DECIMAL_FP_SUPPORT as unsupported.
But configure seems to use cc (i686-apple-darwin10-llvm-gcc-4.2)
instead, which doesn't support fixed-width decimals either, but the
code compiles and links just fine. I suspect it may have something
to do with the llvm backend printed in the version.
Lacking the time to debug this further, the patch fixes the issue by
checking the expected output when fixed-width decimal support is
present and correctly implemented.
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