BUG: Xerbla doesn't get linked in 1.9 on Fedora 21.

Add our python_xerbla to the blasdot sources. That function is
needed for all modules that link to the ATLAS 3.10 libraries, which
in Fedora 21 are located in just two files: libsatlas and libtatlas.

Also make the test for xerbla linkage work better. If xerbla is not
linked the test will be skipped with a message.