python.m4 macro does pass unexpanded argument to get_python_lib()

[Matthias Klose]

seen in 1.10.1.

'${prefix}' and '${exec_prefix}' are passed unexpanded to get_python_lib()), and
the macro then defaults to the hardcoded alternative case. The patch passes the
real path names to get_python_lib(), and only for the case, where the prefix
argument was specified on the command line (the case for a missing prefix
argument is handled inside the get_python_lib() function).

tested the macro with python versions 2.0 up to trunk, and the 3.0 and 3.1 
branches.

  Matthias