Change the default number of CPU threads to one

[ibnesayeed]

While it is great to have a configuration option to set the number of CPU threads to use to build targets in parallel as per #1 and #10, setting the default to anything other than one CPU thread may result in unexpected behavior. When I started using this package for the first time to compile a LaTeX document, the cleanup task didn't seem to be working which was supped to run at the very end. After wasting a few hours I realized that there is a configuration to run tasks in parallel which is causing tasks to run out of order and my LaTeX compilation is taking longer than the cleanup tasks that fines before it had anything to clean, resulting in unnecessary artifacts present in the repo after each build. When I changed that configuration to use only one CPU thread, the problem was vanished.

[Nik-Sch]

This is a problem of your Makefile then. Fix it.