Prepend `-std=c++xy` flag before user-supplied flags

Ansgar Burchardtrequested to merge
feature/do-not-override-cxx-std-flag into master

Appending the automatically detected -std=c++xy flag means any user-provided flag will get silently overridden. Prepending it instead makes it easier for users to override the default option as they no longer have to set DISABLE_CXX_VERSION_CHECK.

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