Move final.hh to std.

DUNE_FINAL is a helper macro for C++11 and should be placed in
std, where now all compatibility has to be placed.
Beautify Doxygen documentation.
Remove superfluous semicolons in checks.

They cased compiler warnings.
[CMake] Add check for HAVE_CONSTEXPR.
[autotools] Rename dune_final.m4 to cxx11_final.m4.
[CMake] Add check for HAVE_KEYWORD_FINAL.