[!354] Properly use SOURCES in dune_add_library

Merge branch 'use-SOURCES-in-dune-add-library' into 'master'

ref:core/dune-grid dune_add_library implicitly treats all unclaimed trailing
arguments as source file names, but explicitly marking those as SOURCE makes
the code more readable.

See merge request [!354]

  [!354]: gitlab.dune-project.org/core/dune-grid/merge_requests/354
6 jobs for master in 39 minutes (queued for 7 seconds)
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debian:9 clang-3.8-14

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ubuntu:16.04 gcc-5-14

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