Fixed K&R/C89 style declarations and other minor changes

Fixed K&R/C89 style declarations; organized headers; set to build with warnings; fixed some of those.

Test/*: Reorganized headers; corrected C89-style declarations. …
Constructions/KeccakDuplex.c: Added an explicit cast
Modes/KeccakHash.c: Added an explicit cast
Added warning flags to Makefile.build
Changed standard to GNU C11