build in parallel in makefile

[yunwei37]

Description

Fixes #173

The build time is:

$ time make build JOBS=8
real    1m47.211s
user    5m14.661s
sys     0m45.855s

which is similar to:

$ cmake -Bbuild -DBPFTIME_ENABLE_UNIT_TESTING=1 -DBUILD_BPFTIME_DAEMON=1 -DCMAKE_BUILD_TYPE:STRING=Debug
$ time cmake --build build --config Debug -j8
real    1m43.163s
user    5m13.930s
sys     0m36.892s

Documents updated in https://eunomia.dev/bpftime/documents/build-and-test/#compilation-for-bpftime

Type of change

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [X] This change requires a documentation update

[Officeyutong]

Would -j$(nproc) be better?

[yunwei37]

Seems there are no easy ways for makefile to get -j number.

In a Makefile, the -j option is used with the make command to specify the number of jobs (commands) to run simultaneously. However, within the Makefile itself, you cannot directly retrieve the number passed to the -j option because it's an argument to the make command and not an environment variable or a Makefile variable.

If you want to act on the -j option within a Makefile, you typically have to pass it as a variable. However, GNU make provides a way to detect if the job server is enabled (which happens when you use -j without specifying the number of jobs) via the .NOTPARALLEL special target. But it does not provide a way to get the actual number of jobs specified by the user.

From GPT4