[krbdev.mit.edu #7812] AES-NI support can break OS X build

[Greg Hudson via RT]

If an OS X machine has yasm installed, the krb5 build will try to use 
AES-NI support, but will fail because the object file format is not ELF.  We either need to build with the correct object file format for OS X or 
disable aesni support on non-ELF platforms.

Additional details at:

http://mailman.mit.edu/pipermail/krbdev/2013-December/011829.html