This brings us up-to-speed with the latest compatibility code from upstream
check git. For completeness, we do all the checks that upstream check does, but
we skip the snprintf/vsnprintf code because it's not straightforward (involves
running code and that is bad for cross-compilation) and not necessary for the
platforms we support anyway.
If someone really wants this, they can uncomment this and copy the relevant
checks from the check git repository.
https://bugzilla.gnome.org/show_bug.cgi?id=775870
Makes it clearer which files are actually used in libcheck and which are used
for cross-platform compatibility. This is going to be especially useful when we
add all the libcompat fallback code that upstream libcheck has which will add
about 6 new files.
https://bugzilla.gnome.org/show_bug.cgi?id=775870
