Platform Assumptions: what does it mean to assume threads?

[Sam Hartman]

So, assuming that a threading library is available and that for example
an application can take advantage of threads seems reasonable.
Similarly, assuming that threads headers for Windows or POSIX threads
are available seems reasonable.

However even when threading is enabled, libk5support tends to use weak
references to threading symbols and not always assume they are linked
into a given application context.

What are we actually talking about doing if we change our threading
assumption?