[Buildsystem] Provide macros for explicitly marking symbols as exported or private

This patch adds a new header visibility.hh with macros DUNE_EXPORT and DUNE_PRIVATE
to mark symbols as exported or private at the ABI level.

While we do not explicitly mark any symbols as hidden, there are certain situations
in which the compiler creates symbols with incorrect linkage, especially for singleton
accessor methods for templated types and their embedded static variables, which involve
the creation of weak symbols.

The linker is then unable to merge those weak definitions, causing either a link-time
failure (when building static libraries) or undefined runtime behaviour (dynamic libraries).

This problem can be avoided by explicitly marking the singleton accessor methods as
DUNE_EXPORT. For symmetry, there is also a DUNE_PRIVATE macro, but while it works, I don't
see it used very much in the short term.