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