README: explain parallel build [no ci]

[JohannesGaessler]

Fixes #7518 .

This PR adds instructions for how to speed up compilation with make and cmake. I am not aware of a portable method for specifying multiple parallel jobs for cmake so I explained how to pass arguments to the internally used build tool instead.

[mofosyne]

Oh btw have you considered recommending the use of nproc instead?

make -j $(nproc)

nproc

Print the number of processing units (normally CPUs) available. More information: https://www.gnu.org/software/coreutils/nproc.

• Display the number of available processing units: nproc
• Display the number of installed processing units, including any inactive ones: nproc --all
• If possible, subtract a given number of units from the returned value: nproc --ignore {{count}}

[JohannesGaessler]

I don't think we should overcomplicate things. The main point of this change is to make users aware that a parallel build is possible at all, and for those users I would just keep the whole thing as simple as possible.

[simonlui]

I am not aware of a portable method for specifying multiple parallel jobs for cmake so I explained how to pass arguments to the internally used build tool instead.

This has been an option since CMake 3.12 though. https://cmake.org/cmake/help/latest/manual/cmake.1.html#build-tool-mode

-j [], --parallel []

New in version 3.12.

The maximum number of concurrent processes to use when building. If is omitted the native build tool's default number is used.

The CMAKE_BUILD_PARALLEL_LEVEL environment variable, if set, specifies a default parallel level when this option is not given.

Some native build tools always build in parallel. The use of value of 1 can be used to limit to a single job.