How to run Makefile tasks parallelly

You have created the following Makefile before:

one:
  echo one

two:
  echo two

parallel: one two

Maybe you were clever and did the following to achieve parallel tasks:

parallel:
  echo one & \
  echo two

But both of them will fail when it comes to long running tasks. But if we run make --help there's an interesting flag we should look into:

-j [N], --jobs[=N]    Allow N jobs at once; infinite jobs with no arg.

With that we can run from the command line to run task one and two:

make -j one two

...or even better, stick it into the Makefile to make it reusable:

one:
  echo one

two:
  echo two

parallel:
  make -j one two