[CMake,release] Remove double slash (//) in path for headercheck.
(cherry picked from commit 518f3fc2eb31b09a833de20960140a7a830d647e)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>
[release] Install dune_cxa_demangle.m4
(cherry picked from commit ccb09bea46d3aec810184ebdf31888755e44cda7)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>