Declare gcc version in rbm.conf

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 legacy/trac#25485 (moved)).

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