msi: Use gcc -dumpversion to get a realible version number
In fact we should always use that. I have not checked when this
option was introduced but it should be long enough evailable.
The gcc manual says:
Print the compiler version (for example, '3.0', '6.3.0' or '7')--and don't do anything else. This is the compiler version *used in filesystem paths and specs*. Depending on how the compiler has been configured it can be just a single number (major version), two numbers separated by a dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version).
We may even drop the other methods we use of swicth for them to
-dumpfullversion