Your criticism is, the compiler is non libre software and can unknowingly to
those, who do the compiling, put unwanted software into the virtualbox
binary?
Yes.
The fact, that the compiler is non libre software does not prohibit the
virtualbox binary from getting a gpl license?
If you receive a binary, you must trust whoever built it. E.g., the Trisquel
project compiles all the packages it distributes so that you only have to
trust the Trisquel development team.
But you do not have to trust anybody: you can build the binary from source.
The process is trustfully as long as every program in the build chain (in
particular the compiler) is free. Well, it is actually more tricky than
that. Ideally we should always have reproducible builds (Debian has been
working on that for years): https://reproducible-builds.org/
Using a libre software compiler, does not stop the virtualbox people from
editing the compiler and introduce to those downloading the binary unwanted
software in the virtualbox binary?
If they are evil, VirtualBox's developers can simply compile and distribute
in a binary form a modified version of their program. However, VirtualBox's
developers may not be evil and the binary you receive from them still be
tainted because the authors of the nonfree compiler are evil.
