cleanup and improve build.py

[mm1ke]

Hi,

This is a simple update for the build.py file which i use for gentoo in https://github.com/gentoo/gentoo/pull/43015

I've made two commits. The first simply removes import platform which was introduces in 591b9f4 but later not required anymore with commit e7ec8ea

The second commit removes COMPILE_FLAGS altogether. There is no reason to force a certain optimization on users. Users should be able to decide themselves how to optimize their packages.

Regards Michael