[build] Reset default CFLAGS in the root CMake file
[build] Reset default CFLAGS in the root CMake file

In Release builds, CMake used -O3 by default, while we want to have
full control on the flags used. Therefore, clearing them is essencial.

vvaltchev/experimentOs,vvaltchev/experimentOs,vvaltchev/experimentOs,vvaltchev/experimentOs