Now this is a real problem.
I've installed apt-build, because I wanted to see if I can build optimised packages on Debian. The install itself was sweet, it asked me in a Dialog box about march and mcpu and i selected them. The problem is that I want more options, sort of Gentoo or BSD "make.conf" style.
I know that there are no USE flags, but the problem is this:
How can i set the behaviour of gcc and ld? There must be a way to do this in a centrallised fashion. I just want apt-build to use gcc with my options, like --ffast-math. The apt-build.conf has an options sections, but it seems that it is for apt-build's options.
It it possible to set them, or do I need blackmagic to do it?
Thank you.

Yes, is posible.
Options sections is for specify options for gcc, for example my apt-build.conf:

build-dir = /var/cache/apt-build/build
repository-dir = /var/cache/apt-build/repository
Olevel = -O2
mcpu = -mcpu=pentium4
options =" -funroll-loops -mmmx -msse2"
make_options =""

or you can personally edit debian/rules in source directory to add you options for gcc

Grrrreat!
Now another related problem: Is there a way to find out the compiler/linker options of a package after it's compiled? Or such information isn't kept in a .deb?

I think it`s impossible, for what you want know this information?

First of all, to have some reference (original flags and such), and secondly because I'm a non-believer I would like to check that apt-build actually uses my settings...I was thinking about using some crazy setting like O9 to see if the apps keep crashing, but apps can crash even with O0 so that's not a test...I'm going to keep thinking about something like this. Anyone who has an idea of a sort of reliable test is welcome (just to check that any of the options work, doesn't matter which).