Re-merge "Allow disabling the pthread support check"

See #6: this PR was
accidentally merged as a merge commit, which got lost in the recent
refactor+squash for 184. This change cherry-picks the actual commit from
Stefano in that PR.

---

In cases where this code is utlized only for the libriers in it
(e.g. vTPM emulation in Coconut SVSM), we do not need to compile
the simulator.

So it would be convenient to have the ability to disable pthread
checking, which in that case is not necassary since it is used only by
the simulator. Also in Coconut SVSM we don't have pthread support, so
it's also a requirement and for now we have our own fork to disable
this check.

By default pthread checking remains enabled, so there is no change.
`--disable-pthread` is added to the configure to support this new use
case.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

Trusted Computing Group (TCG) TPM Specification 1.83 Release referenc…

…e code. (#262)

Co-authored-by: Brad Litterell <bradlit@microsoft.com>