tests: fix a typo in a type name, caught by clang
unifdefall: use cc -E -dM instead of cpp -dM

This is a workaround for Mac OS X, in which ccp invokes gcc
with the -traditional-cpp option which disables C99 variadic
macros. Unfortunately stdio.h requires variadic macro support
so real programs, as well as the tests, are likely to break.

I am not sure if this is the right fix. It is marginally closer to
POSIX, since it has c99 -E but not cpp. But POSIX also lacks cc and
the -dM option.
tests: locale-independent diffs in multi-file tests

We want to sed away file mtimes from the diff output but date/time
format may depend on platform and locale.
(Noticed on OpenBSD 5.1)
tests/multimissing: avoid platform-dependent error message

Reported-by: Ruediger Meier <ruediger.meier@ga-group.nl>