Fix build failure with Python installed in non-system location.

Fix build failure with Python installed in non-system location.

The debugger fails to build when configure with --python-python=<path>
where <path> is a non-system location.  The reason is a warning made
fatal due to the definition of _XOPEN_SOURCE inside pyconfig.h.  This
is exactly the same problem as with _POSIX_C_SOURCE, handled in
python-internal.h as follow:

| /* /usr/include/features.h on linux systems will define _POSIX_C_SOURCE
|    if it sees _GNU_SOURCE (which config.h will define).
|    pyconfig.h defines _POSIX_C_SOURCE to a different value than
|    /usr/include/features.h does causing compilation to fail.
|    To work around this, undef _POSIX_C_SOURCE before we include Python.h.  */
| #undef _POSIX_C_SOURCE

This patch fixes this problem the same way.

2010-06-30  Joel Brobecker  <brobecker@adacore.com>

        * python/python-internal.h (_XOPEN_SOURCE): Undefine before
        including Python.h.