Apple updated the Xcode command line tools today, which upgraded clang to Apple LLVM version 5.1 (clang-503.0.38) (based on LLVM 3.4svn) and enabled a feature in clang that causes unknown compiler flags to hard error rather than warn. The error is accompanied by an amusing/frustrating note:

clang: error: unknown argument: '-mno-fused-madd' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future

Apparently the future is here.

This is a serious problem because several common gcc flags are not supported under clang (most notably -mno-fused-madd).

For now you can work around the issue using

export ARCHFLAGS="-Wno-error=unused-command-line-argument-hard-error-in-future"

but be aware that this will stop working in a future clang.