build: compile take too much memory and might freeze the host machine

## Brief

@Shouren have made some test for that:
https://github.com/4paradigm/OpenMLDB/pull/870#discussion_r766343266

turns out compile may easy to down if
- host memory is low, say 4G
- enable parallel build, current default to `$(nproc)`

especially compile targets like `avg_by_category_def.cc`

## Other

Should documented minimal requirements of compilaion