cppcheck: allow for extra user and developer flags

Make cppheck calls use flags variables `$(AM_CPPCHECKFLAGS)` (for
developer flags) and `$(CPPCHECKFLAGS)` (for user flags).

This allows both a developer to add specific flags needed to cppcheck
to behave appropriately for the checked code, as well as the users to
specify flags, like e.g. the number ob concurrent jobs.
cppcheck: fix rule portability

`$^` automatic variable is not portable, so explicitly use `$(srcdir)`.

This fixes `make check` with BSD make.
spellcheck: fix checking with cppcheck

cppcheck tests all preprocessor conditionals, but when the code tests
whether one is defined or not it tests simply with it defined or not,
but always to an empty value.  So, code testing whether a macro is
defined, but then relying on the macro expanding to a non-empty value,
cppcheck might very well detect a syntax error.

So, explicitly tell cppcheck to only test the SCE_PAS_DEFAULT=0 case
so it doesn't incorrectly test the SCE_PAS_DEFAULT= case.