How do I get rid of the -fvtable-gc flag?

How do I get perminantly rid of the -fvtable-gc flag? I can remove it from the project make files, but it re-appears when I rebuild the system library.

I found a file "rules.mak" which appears to be an attempt to remove the flag, that doesn't seem to work.

It would be very nice to know where the default flags come from since I also have a need to modify other flags as well