Manually find the site packages directory

On some systems the prefix is incorrectly set to /usr/local.

    * Makefile.in: Regenerated
    * configure: Regenerated
    * configure.ac: Create a PYTHON_SITEPACKAGES variable to tell make
        where to put the mageec module
    * doc/Makefile.in: Regenerated
    * gcc_plugin/Makefile.in: Regenerated
    * include/Makefile.in: Regenerated
    * mageec/Makefile.am: Change the install hooks to place library
        correctly
    * mageec/Makefile.in: Regenerated
    * ml/C5/Makefile.in: Regenerated
    * ml/Makefile.in: Regenerated
    * testsuite/Makefile.in: Regenerated
    * testsuite/gcc_plugin.tests/Makefile.in: Regenerated
    * testsuite/mageec.tests/Makefile.in: Regenerated