Starting with Python3, standard Python installs may have additional ABI
flags attached to include directories and library names. As of 3.2, the
following flags are in the configure file:

    d -> --with-debug
    m -> --with-pymalloc
    u -> --with-wide-unicode

Python 3.3 seems to no longer have --with-wide-unicode. Hopefully Python
will ensure that the possible flags always show up in a stable order.
The 'd' flag is ignored since the debug library is considered separate.

There is still the problem where ABI flags cannot be specified in
find_package since the letters confuse the version comparator.