Use C++11 std::thread instead of pthread.

[mgautierfr]

This is necessary as libzim now does the same and we don't install wrapper pthread library on windows CI.

[codecov]

Codecov Report

Merging #445 (24b2e6e) into master (f5d9a37) will decrease coverage by 0.04%.
The diff coverage is 15.38%.

@@            Coverage Diff             @@
##           master     #445      +/-   ##
==========================================
- Coverage   59.77%   59.73%   -0.05%     
==========================================
  Files          51       50       -1     
  Lines        3416     3410       -6     
  Branches     1721     1725       +4     
==========================================
- Hits         2042     2037       -5     
+ Misses       1371     1370       -1     
  Partials        3        3              

Impacted Files	Coverage Δ
include/downloader.h	0.00% <ø> (ø)
src/entry.cpp	50.00% <ø> (ø)
src/server/internalServer.cpp	86.39% <ø> (ø)
src/subprocess_unix.cpp	0.00% <0.00%> (ø)
src/tools/regexTools.cpp	100.00% <100.00%> (ø)

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