Make sure PYTHON is absolute.

* m4/pypath.m4: Here.  This is needed because PYTHON is used as
she-bang line in scripts.

m4/pypath.m4

 AC_DEFUN([adl_CHECK_PYTHON],
  [AM_PATH_PYTHON([2.0])
+  case $PYTHON in
+    [[\\/$]]* | ?:[[\\/]]* );;
+    *) AC_MSG_ERROR([The PYTHON variable must be set to an absolute filename.]);;
+  esac
   AC_CACHE_CHECK([for $am_display_PYTHON includes directory],
     [adl_cv_python_inc],
     [adl_cv_python_inc=`$PYTHON -c "import sys; from distutils import sysconfig;]