When python C extensions are built distutils uses the same compiler / linker flags that were used to build the python interpreter itself. The interpreters provided by Apple in OSX (python 2.5, 2.6, and 2.7) were built with -mno-fused-madd. Clang doesn't support that flag and Apple has never fixed their interpreter builds to not use it. This hasn't been a problem in the past since previous versions of clang ignored unrecognized flags. Clang 3.4 changes the situation by erroring out when it encounters unrecognized flags.

In classic Apple fashion no updated interpreters were provided with the Xcode 5.1 update so installation of packages with C extensions (e.g. pymongo, greenlet, gevent, simplejson, etc, etc, etc) fail with an error similar to this:

clang: error: unknown argument: '-mno-fused-madd' [-Wunused-command-line-argument-hard-error-in-future]

PyMongo 2.7 will ship with a hack to work around the problem but in the meantime the following workarounds exist:

pip:

$ ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future pip install pymongo
 
or

$ CFLAGS=-Qunused-arguments pip install pymongo

easy_install:

$ ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future easy_install pymongo

or

$ CFLAGS=-Qunused-arguments easy_install pymongo

If your python interpreter was built without -mno-fused-madd you won't have this problem to begin with. You can check what cflags were used to build python on OSX with the python-config tool:

$ python-config --cflags

See this stackoverflow thread for ongoing discussion:
http://stackoverflow.com/questions/22313407/clang-error-unknown-argument-mno-fused-madd-psycopg2-installation-failure

Xcode 5.1 release notes. See the "Compiler" section were this breakage is documented:
https://developer.apple.com/library/ios/releasenotes/DeveloperTools/RN-Xcode/Introduction/Introduction.html