Mingw doesn't implement it. So, we should not use it.
I suppose that it was working before because mingw package for debian trusty
simply no provides a "thread" header.
We may face to include the native "thread" header.
The `common` name is from the time where kiwix was only one repository
for all the project (android, desktop, server...).
Now we have split the repositories and kiwix-lib is the "common" repo,
the "common" directory is somehow nonsense.
By default, we are searching in the PATH env var.
However, with an appImage, the executable directory is not in the PATH,
so we have to use an absolute path if we can.
If we cannot find the aria2c executable in the executable directory let's
try to use the system one.