CMake: Bugfix: Replace all "-" by "_" for some internal variable name

ENABLE_IF_SUPPORTED incorrectly exported invalid (cpp) macro definitions
containing a dash "-". Fix this by replacing all dashes with underscores
"_".
CMake: Bugfix: Correctly detect unsupported flags in case of clang

The clang compiler frontend is very conservative in not issuing an error
for unsupported compile flags but that are valid for gcc. Therefore, also
supply "-Werror" for the clang compiler in all 'ENABLE_IF_SUPPORTED' tests.
Add a configure time check for a bug in icc

icc-14*, icc-15* does not play nicely with enabled -std=c++11 and
boost::signals2, disable C++11 support in this case.

[1] https://software.intel.com/en-us/forums/topic/515966
Merge pull request #179 from tamiko/cmake_bugfixes

This is a series of small bugfixes for clang and icc configuration:
comment on spurious eigenvalues in Step-36
Reindent and fix the issues discussed in the original pull request.
Rename a couple of variables and extend discussion.
Extend discussion about spurious eigenvalues.
Add a changelog entry.
Merge pull request #180 from bangerth/comment-on-spurious-eigenvalues

Comment on spurious eigenvalues