Fix for PR automake/291:

Fix for PR automake/291:
* automake.in (macro_define): Catch variable names starting
with `_'.
(handle_installdirs): Rename _am_installdirs to	am__installdirs.
* lib/am/data.am, lib/am/install.am, lib/am/libs.am,
lib/am/lisp.am, lib/am/ltlibs.am, lib/am/mans.am,
lib/am/progs.am, lib/am/python.am, lib/am/scripts.am,
lib/am/texinfos.am: Likewise.
* tests/canon5.test: Use -Wno-portability.
* tests/vars2.test: New file.
* tests/Makefile.am (TESTS): Add vars2.test.