Compat: pthread.h shim for FreeRTOS

[projectgoav]

As discussed in #960.

Assumes that anyone wishing to build for FreeRTOS will define FREERTOS and ensure that the FreeRTOS headers are added to the include path at FreeRTOS/___

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pthread_once

I wasn't sure if this has to be thread-safe. Can easily adjust this if required.

Syslog Support

I had to adjust the compat/syslog.h slighty to account for more than just Windows not providing syslog. Please let me know if there is a better/cleaner/more LibreSSLy way of doing things like this.

I liked making sure of the already defined NO_SYSLOG which is used as part of crypto/compat/r_syslog.c hence why this was picked.

Sockets / Networking

We currently use the LWIP networking stack. My plan was to submit another PR with a compatibility shim for that. As a result, I don't know the status of building for the built-in FreeRTOS stack. It's my understanding it follows the standard socket API so might just work (TM)...

[busterb]

Hi @projectgoav do you have build / test instructions for this branch?