Small bugfixes wrt DEAL_II_HAVE_AVX512

 - Use the same logic as for DEAL_II_HAVE_(SSE|AVX)
 - Update documentation
Refactor openmp-simd setup

 * The check for support of -(f)openmp-simd is now done in
   checks/check_01_cpu_features.cmake and guarded by
   DEAL_II_ALLOW_PLATFORM_INTROSPECTION

   The guard is important in order to allow for platform independent
   compilations (as required by Linux distributions)

   Furthermore, this allows for manual override via
   DEAL_II_HAVE_OPENMP_SIMD.

 * The pragma statement is refactored into a macro defined in
   config.h.in. With this, no -Wno-unknown-pragmas workarounds are
   necessary and picky compilers that choke on "#pragma" are happy.
Merge pull request #126 from tamiko/refactor_openmp_simd_setup

Refactor openmp simd setup
Fix up a few places that accidentally didn't get changed when downsampling data.
Reference the correct picture.
Fix language.
Merge pull request #130 from bangerth/fix-up-step-53-2

Fix up step 53. This was supposed to be addressed by #127 but the merge of that issue apparently encountered some problems. Here's a respin.
Move a couple of entries in the changelog to their correct place.
Merge pull request #131 from bangerth/sort-changelog

A couple of entries had ended up in the wrong place (general changes instead of specific changes). While it's debatable where exactly the line is, these items should probably be in the second category. Move them there.