Fix for #416, compilation warnings for implicit-falltrough on gcc7.1

[furkanusta]

After checking external/rapidjson/internal/regex.h, I've decided to copy same pragmas instead of relying on the inclusion of regex.h because regex.h pops back the pragmas at the end of the file (although not for gcc), and thus it would be better to change these pragmas per-file basis.

It passes all the tests with gcc7.1.1 on my machine and a simple test code generates no warnings on clang4.0

However whole project fails to build on clang4.0 giving an unrelated error. I will check it as well and send a pull request if I can.

And a suggestion: maybe we can define a macro like CEREAL_FALLTHROUGH that expands to C++17 counterpart if it is defined and to nothing otherwise.

[furkanusta]

Ignore this patch for now, as it seems that this one fixes the problem for gcc7.1 but gives an error for previous versions since implicit-fallthrough is not available on them. I'll fix it and send an another patch.