Use new @pthread library from toktok-stack for Windows compat.

[iphydf]

See TokTok/toktok-stack#38.

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This change is 

[robinlinden]

Reviewed 1 of 1 files at r1.
Reviewable status: 0 of 1 approvals obtained

[codecov]

Codecov Report

Merging #1197 into master will decrease coverage by 0.2%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1197     +/-   ##
========================================
- Coverage      83%   82.8%   -0.3%     
========================================
  Files          82      82             
  Lines       14645   14646      +1     
========================================
- Hits        12167   12129     -38     
- Misses       2478    2517     +39

Impacted Files	Coverage Δ
auto_tests/network_test.c	90.3% <0%> (-7.3%)	⬇️
toxcore/TCP_client.c	64.3% <0%> (-2.8%)	⬇️
toxcore/TCP_server.c	75% <0%> (-2.8%)	⬇️
toxcore/net_crypto.c	92.9% <0%> (-1.5%)	⬇️
toxcore/LAN_discovery.c	84.9% <0%> (-1%)	⬇️
toxav/msi.c	63.9% <0%> (-0.3%)	⬇️
toxcore/Messenger.c	86.5% <0%> (-0.2%)	⬇️
toxcore/onion_client.c	95.3% <0%> (ø)	⬆️
toxcore/friend_connection.c	94.8% <0%> (+0.2%)	⬆️
toxav/toxav.c	67.3% <0%> (+0.3%)	⬆️
... and 1 more

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