Does not work correctly if user uses MAKEFLAGS to enable parallel make.

Many users do things like export MAKEFLAGS="-j$(nproc)", which tells make to use parallel jobs to speed up builds on a multi-core machine.

However, the Makefile that gdk calls is written with many dependencies missing, meaning that parallel make may call jobs out-of-order. For instance, when running gdk install, it will try to copy .yml.example files to just .yml before the git repository containing the .yml.example file has finished cloning.

Normally, I would suggest explicitly declaring the dependencies in the Makefile, but given that it seems to be being phased out anyway (#166), it may be more appealing to just stick .NOTPARALLEL: somewhere in the Makefile.

mentioned in merge request !263 (closed)

@lukeshu Thanks for the suggestion! I makes sense to improve that indeed!

MakeFlags are generally messed up. Not only here in GitLab. Everywhere

While some projects are broken, setting MAKEFLAGS=-jN is generally safe (though other flags obviously aren't). Empirically, a large percentage of the packages for Parabola GNU/Linux-libre are built with MAKEFLAGS=-j2 with no problem.