Don't pass the cpu flags detected by Vc to the compiler

Vc detects CPU flags based on the name of the CPU. This can be a problem in virtualization environment, when the name of the CPU is visible, but not all features are enabled in the guest.

Also, enabling all detected features in the the compiler makes it difficult to impossible for the user to force his own feature flags, e.g. to cross-build generic executables on hosts with many features to be run later on hosts with fewer features.

Closes: docker/ci#9

WIP:

• properly test, in particular in the light of docker/ci!46 (merged) being applied to the ci images
• ansgar proposed getting rid of all the Vc_COMPILE_FLAGS, too. Try that: !679 (closed) (redifined !679 (closed) as an add-on to this one, so it does not stop me from merging this one)
• changelog entry (deferred to #163 (closed))