Regression test: libccpp has both C and C++ sources. The executable only
has C sources. It should still link using the C++ compiler. When using
both_libraries the static has no sources and thus no compilers,
resulting in the executable linking using the C compiler.
https://github.com/Netflix/vmaf/issues/1107
This commit introduces a new type of `HoldableObject`: The
`SecondLevelHolder`. The primary purpose of this class is
to handle cases where two (or more) `HoldableObject`s are
stored at the same time (with one default object). The
best (and currently only) example here is the `BothLibraries`
class.