I am running into a problem related to this. I am attempting to cross
compile extensions but Fedora includes -march in the OPT variable. Since
there is no way to exclude the OPT values the build fails.

It seems that forcing OPT to stay the same is a disservice generally
speaking since as Marien said you potentially get extra conflicting
flags (-O seems like a stand-out since that could mess with gdb).

2.6 and 3.1 say:
        if 'CFLAGS' in os.environ:
            cflags = opt + ' ' + os.environ['CFLAGS']
            ldshared = ldshared + ' ' + os.environ['CFLAGS']

There is no ability to override opt. I'd be willing to put together a
patch to allow opt to be overridden by an environment variable if I
could get some support for this from a core maintainer.

I am also open to other suggestions about how to get around this. I
potentially have time to put into fixing this (or the more general cross
compile issue).