Okay so to close the question:

The Problem was that opencv needs both `Lapack` with the normal `blas` and `cblas` if you are using the `ceres-solver`. However, the opencv build only links `cublas`, which apparently lacks support for some needed functions of cblas.

One solution to this Problem was to manually link `cblas` by adding 
`CMAKE_EXE_LINKER_FLAGS=-cblas` to the `cmake` call in the PKGBUILD file.

It is probably possible to circumvent this problem altogether by building all dependencies manually with forced `cublas` support. However, that is tedious and not always possible since `cublas` is only a partial port. 

Thanks again to [Philippos](https://unix.stackexchange.com/users/216004/philippos), who helped me narrrow the problem down.