flag for selecting a specific gcc toolchain version

i noticed that there seems to be no way to select a particular gcc toolchain version, am i correct ?
–gcc-toolchain seems not to be very granular as it expects sth like /usr and then automatically picks the latest version.

Currently, as a workaround i’m mimicking /usr using symlinks excluding all the gcc versions i don’t want.
I’ve posted the question and answered it on stackoverflow: http://stackoverflow.com/a/41982109/3053638

Assuming there is no better way, i would love to contribute a patch.
Can anyone give me a hint where the best place is to integrate this functionality?