configure: try to make all PYTHON variables point to the same thing

Change-Id: Ifa6a22717d6911b1f5dd816144d3ee95a09879a3
diff --git a/configure.ac b/configure.ac
index a70bfd2..8451b74 100644
--- a/configure.ac
+++ b/configure.ac
@@ -7499,6 +7499,10 @@ if test $enable_python = system; then
               AC_MSG_ERROR([no usable python found])])
        test -n "$PYTHON_CFLAGS" && break
    fi
    # let the PYTHON_FOR_BUILD match the same python installation that
    # provides PYTHON_CFLAGS/PYTHON_LDFLAGS for pyuno, which should be
    # better for PythonTests.
    PYTHON_FOR_BUILD=$PYTHON
fi

dnl By now enable_python should be "system", "internal" or "no"