I recently tried installing lxml for use within an App Engine project on OS X Mavericks only to be hit with an error message from clang.

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

error: command 'cc' failed with exit status 1

The clang compiler distributed with version 5.1 of Xcode tightened up some restrictions and turned compiler warnings into hard errors. To disable this you need to add a specific flag to ingore these warnings when installing affected packages.

ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future pip install lxml