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

Look at the build configuration, likely in CMakeLists.txt, to understand the parallel build settings. Examine the problematic compilation target 'avg_by_category_def.cc' and similar files to gauge memory usage. The goal is to adjust build parallelism or document minimal memory requirements to prevent host freezes during compilation.

Written by the indexing model from the issue text.