Fix for PR automake/398:

Fix for PR automake/398:
* m4/python.m4: Do not call AC_PATH_PROGS if $PYTHON is already
set.  Display `none' instead of `:' and $PYTHON is set to `:'
when no suitable interpreter is found.  Honor ACTION-IF-FOUND and
ACTION-IF-NOT-FOUND.
* automake.texi (Python): Document ACTION-IF-FOUND and
ACTION-IF-NOT-FOUND.
* tests/python4.test, tests/python5.test, tests/python6.test,
tests/python7.test, tests/python8.test, tests/python9.test: New
files.
* tests/Makefile.am (TESTS): Add them.
Report from Per Cederqvist.