build: Allow comments to suppress case fallthrough warnings

By default -Wextra turns on warnings for case fallthoughs, VPR includes
some of these which are intentional and commented. This allows GCC to
suppress warnings in cases where the comment makes it clear it is
intentional.

Author: kmurray

CMakeLists.txt

@@ -141,6 +141,7 @@ else()
         "-Winit-self"                   #Warn about self-initialization
         "-Wcatch-value=3"               #Warn when catch statements don't catch by reference
         "-Wextra-semi"                  #Warn about redudnant semicolons
+        "-Wimplicit-fallthrough=3"      #Warn about case fallthroughs, but allow 'fallthrough' comments to suppress warnings
         #GCC-like optional
         #"-Wsuggest-final-types"         #Suggest where 'final' would help if specified on a type methods
         #"-Wsuggest-final-methods"       #Suggest where 'final' would help if specified on methods