Explicitly create output directories for flex/yacc output.

When out-of-source build with '--disable-dependency-tracking' is
performed, the 'lib/route' and 'lib/route/cls' directories required for
flex/yacc output do not exist. As a result, the flex/yacc calls fail
with ENOENT.

Create the necessary directories explicitly via $(MKDIR_P)
in the flex/yacc rules to guarantee proper out-of-source and in-source
build.
Use paths relative to srcdir in setup.py.

Currently, setup.py is created by configure in builddir while source
files (.py and .i) reside in srcdir. Adjust paths in setup.py
appropriately to make it possible to perform an out-of-source build.

This is far from perfect but it mostly works. Python files are copied
from srcdir, and swig *.i files are read from there. However, swig
output files are created in srcdir rather than builddir, and distutils
copies '..' literally when constructing '.o' paths. As a result,
'../python/foo.i' would end up being compiled to
'build/temp*/../python/foo.i'.

The alternative would be to copy '*.i' files to builddir before
proceeding with the build, either in Makefile or through creating
additional distutils command.
Merge branch 'mgorny-master'

https://github.com/thom311/libnl/pull/58

Signed-off-by: Thomas Haller <thaller@redhat.com>