Use tinycthread on all platforms

[wch]

Fixes #77. This PR makes it so later stops trying to use C11-style threads.

It changes the function names in tinycthread so that they don't conflict with (and inadverantly link to) functions with the same name from C11-style threads.

Installation can be tested by building the Fedora 29-Anaconda Docker image as described in #77, then running the following:

docker run --rm -ti fr /bin/bash
yum install -y unzip
wget https://github.com/r-lib/later/archive/remove-tinycthread.zip
unzip remove-tinycthread.zip
R CMD build --no-build-vignettes later-remove-tinycthread
R CMD INSTALL later_0.7.5.9001.tar.gz

For some reason, R CMD check doesn't work, but I think it's because there's some missing system dependencies.

[root@f07d3a24fdad /]# R CMD check later_0.7.5.9001.tar.gz 
Error in system(paste(which, shQuote(names[i])), intern = TRUE, ignore.stderr = TRUE) : 
  error in running command
Execution halted

[QuLogic]

This name (thrd_t) still exists even though it's listed here as bad. Pretty sure that means that THREADS_H_ERROR becomes the name of the type, which does mean the wrong type name is avoided, but I'm not sure that's what was intended.

[QuLogic]

On a related note, names ending in _t are reserved for future type names and should not be used. Technically, tinycthread was in violation of this already...