bug#11762: AM_PATH_PYTHON regression in automake 1.11.3

[Robert Collins]

 AM_PATH_PYTHON used to generate install logic like:
 if test -z "$(DESTDIR)"; then \
    PYTHON=$(PYTHON) $(py_compile) --basedir "$(demodir)" $$dlist; \
 else \
   PYTHON=$(PYTHON) $(py_compile) --destdir "$(DESTDIR)" --basedir
"$(demodir)" $$dlist; \
 fi; \

 Now however, it generates it without the test -z guard, which makes
regular 'make install', and 'make distcheck' fail:
 make distcheck
 ....
 ../py-compile: Missing argument to --destdir.

I reported this in Ubuntu as bug 997456 -
bugs.launchpad.net/bugs/997456; we've patched it locally by reverting
the previous patch.

Cheers,Rob