This patch removes an extra header install command for tests. It
originally came when copying dune/common/CMakeLists.txt to
dune/common/std/CMakeLists.txt. Sorry for the noise.

The majority of developers favor individual headers for (emulation of) C++11
features over collecting them in a single header cxx11.hh. This patch
renames cxx11.hh to constexpr.hh and moves it into the subdirectory std.
This is the place most developers would like these headers in (see
http://users.dune-project.org/doodles/4).