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
Add BOOST.fusion as apparently we need that in boost/math/special_functions/erf.hpp.
Add a test to avoid this problem from happening again in the future.
Circumvent a bug in Trilinos 11.10

If we avoid calling GlobalAssemble when copying from an Epetra_FECrsMatrix that uses a
non-local matrix (enabled for Trilinos 11.9 and greater), we do not run into some bad
code in Trilinos causing a segfault. At this point of the code, it is enough if we only
call FillComplete because there should be no communication after all.
Merge pull request #182 from kronbichler/master

Circumvent a bug in Trilinos 11.10
Merge pull request #181 from bangerth/add-fusion-to-boost

Add fusion to boost
MPI_Allreduce is apparently unable to accept input and output arguments at the same location.

Fix this by determining when &input=&output and in that case pass MPI_IN_PLACE.
Add a bunch of missing tests for Utilities::MPI::max.
Merge pull request #183 from bangerth/harden-mpi-reductions

Harden mpi reductions
Provide better documentation on ghosted vectors.

This includes a discussion of what ghosted vectors are and what their
semantics are.
Merge pull request #177 from bangerth/document-ghosted-vectors

Provide better documentation on ghosted vectors.