* tests/headers.at (AT_TEST_CPP_GUARD_H): Don't assume that

* tests/headers.at (AT_TEST_CPP_GUARD_H): Don't assume that
"$CC -E foo.h" is allowed, as this doesn't work with the Portland
Group compiler.  Instead, use "$CC -E bar.c".  Include the .h
file twice in the grammar, as an extra check.

* tests/input.at (Torturing the Scanner): Surround the
backslash-newline tests with "#if 0", to make it less likely that
we'll run into compiler bugs.  Bring back solitary \ inside
comment, but add a closing comment to work around HP C bug.  Don't
test backslash-newline in C character constant.  This should fix
the input.at bug reported by Nelson H. F. Beebe in
<http://mail.gnu.org/pipermail/bug-bison/2002-November/001893.html>.