correct python libdir to avoid configure warnings for x86_64

DEBUG: checking whether Python support is requested... autodetect
DEBUG: checking for a Python interpreter with version >= 2.3... python
DEBUG: checking for python... /usr/bin/python
DEBUG: checking for python version... 2.7
DEBUG: checking for python platform... linux2
DEBUG: checking for python script directory... ${prefix}/lib/python2.7/site-packages
DEBUG: checking for python extension module directory... ${exec_prefix}/lib64/python2.7/site-packages
DEBUG: sed: can't read /usr/lib/python2.7/config/Makefile: No such file or directory
DEBUG: sed: can't read /usr/lib/python2.7/config/Makefile: No such file or directory
DEBUG: checking for PYGTK... yes
DEBUG: checking for pygobject defs... /usr/share/pygobject/2.0/defs
DEBUG: checking for pygtk defs... /usr/share/pygtk/2.0/defs
DEBUG: checking for pygobject codegen... /usr/bin/python /usr/share/pygobject/2.0/codegen/codegen.py
DEBUG: checking for pygobject h2def... /usr/bin/python /usr/share/pygobject/2.0/codegen/h2def.py