Declare gcc version in rbm.conf

[sukhbir]

We declare the gcc version (currently set to 6.4.0) in multiple places:

projects/gcc/config:version: 6.4.0

projects/mingw-w64/config:  gcc_version: 6.4.0

We should probably define it in rbm.conf instead and then refer to that. (This may also be relevant for #25485).

[sukhbir]

Quick fix:

​https://github.com/azadi/tor-browser-build-1/tree/bug-26877

[boklm]

I think the two gcc versions used in projects/gcc and projects/mingw-w64 are independent, and could sometimes be two different versions. So I'm not sure we should define it in rbm.conf. But if we define it in rbm.conf, we should then probably also define the sha256sum there too.

[gk]

Hm, I am inclined to think it makes sense to keep those versions separated. Assuming we hit a toolchain bug for Linux which needs to get handled by using a newer compiler it does not seem smart to suddenly ship a new toolchain for Windows, too (or vice versa), especially if the stable series is affected. Thus, I think we should be conservative here and not have both linked together. On the other hand, the question is how often this actually happens and whether we could not work around that ad hoc without the need for bumping the compiler version of the unaffected platform. So, I don't feel too strongly here.

Do we know why we had this separation in the past? I think we did not have that back in the tor-browser-bundle days.

[gk]

Add magic gitlab keyword.