When malloc is not available, this will set #define malloc rpl_malloc
which is implemented only inside libcheck, and not everything will link
to libcheck.
We don't really need to care too much about how malloc is implemented
and we don't care about platforms that don't implement malloc.
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
