This pull request is trying to fix a problem about security

When installing something, users take it for granted to give command higher permission. I think most users use sudo xmake install instead of xmake install, because installing will put files into paths like /usr/local and root permission is required.

That is the problem. I found xmake will always build the target before installing even if it has been built before. And the building is a latent danger. For example, a bad man replace the compiler like gcc, and while installing, the fake gcc will get root permission to do something bad.

There are two solutions to avoid this. One is to set permission lower by process self before actual installation. Another is to do least things with high permission. My solution is to let task 'install' avoid building and use 'safe_config' which will not reconfig instead of 'config'. When users directly do task 'install' without building before, they will get a note to do building.

Also, except security, the build by root will generate result files with wrong owner. On my computer, after installation, I couldn't even delete 'build' dir without sudo

My solution may be not perfect enough. Please do not merge easily. But the secure problem must be fixed.