[lib] Make lib compatible with -Wfall-through excepting legacy

Switch to a macro `ZSTD_FALLTHROUGH;` instead of a comment. On supported
compilers this uses an attribute, otherwise it becomes a comment.

This is necessary to be compatible with clang's `-Wfall-through`, and
gcc's `-Wfall-through=2` which don't support comments. Without this the
linux build emits a bunch of warnings.

lib/compress/huf_compress.c

@@ -596,16 +596,19 @@ HUF_compress1X_usingCTable_internal_body(void* dst, size_t dstSize,
     n = srcSize & ~3;  /* join to mod 4 */
     switch (srcSize & 3)
     {
-        case 3 : HUF_encodeSymbol(&bitC, ip[n+ 2], CTable);
-                 HUF_FLUSHBITS_2(&bitC);
-		 /* fall-through */
-        case 2 : HUF_encodeSymbol(&bitC, ip[n+ 1], CTable);
-                 HUF_FLUSHBITS_1(&bitC);
-		 /* fall-through */
-        case 1 : HUF_encodeSymbol(&bitC, ip[n+ 0], CTable);
-                 HUF_FLUSHBITS(&bitC);
-		 /* fall-through */
-        case 0 : /* fall-through */
+        case 3:
+            HUF_encodeSymbol(&bitC, ip[n+ 2], CTable);
+            HUF_FLUSHBITS_2(&bitC);
+            ZSTD_FALLTHROUGH;
+        case 2:
+            HUF_encodeSymbol(&bitC, ip[n+ 1], CTable);
+            HUF_FLUSHBITS_1(&bitC);
+            ZSTD_FALLTHROUGH;
+        case 1:
+            HUF_encodeSymbol(&bitC, ip[n+ 0], CTable);
+            HUF_FLUSHBITS(&bitC);
+            ZSTD_FALLTHROUGH;
+        case 0: ZSTD_FALLTHROUGH;
         default: break;
     }
 

lib/compress/zstd_compress.c

@@ -2977,7 +2977,9 @@ static size_t ZSTD_writeFrameHeader(void* dst, size_t dstCapacity,
     if (!singleSegment) op[pos++] = windowLogByte;
     switch(dictIDSizeCode)
     {
-        default:  assert(0); /* impossible */
+        default:
+            assert(0); /* impossible */
+            ZSTD_FALLTHROUGH;
         case 0 : break;
         case 1 : op[pos] = (BYTE)(dictID); pos++; break;
         case 2 : MEM_writeLE16(op+pos, (U16)dictID); pos+=2; break;
@@ -2985,7 +2987,9 @@ static size_t ZSTD_writeFrameHeader(void* dst, size_t dstCapacity,
     }
     switch(fcsCode)
     {
-        default:  assert(0); /* impossible */
+        default:
+            assert(0); /* impossible */
+            ZSTD_FALLTHROUGH;
         case 0 : if (singleSegment) op[pos++] = (BYTE)(pledgedSrcSize); break;
         case 1 : MEM_writeLE16(op+pos, (U16)(pledgedSrcSize-256)); pos+=2; break;
         case 2 : MEM_writeLE32(op+pos, (U32)(pledgedSrcSize)); pos+=4; break;
@@ -4332,7 +4336,7 @@ static size_t ZSTD_compressStream_generic(ZSTD_CStream* zcs,
                 zcs->outBuffFlushedSize = 0;
                 zcs->streamStage = zcss_flush; /* pass-through to flush stage */
             }
-	    /* fall-through */
+	    ZSTD_FALLTHROUGH;
         case zcss_flush:
             DEBUGLOG(5, "flush stage");
             assert(zcs->appliedParams.outBufferMode == ZSTD_bm_buffered);

lib/compress/zstd_compress_internal.h

@@ -486,7 +486,7 @@ MEM_STATIC int ZSTD_disableLiteralsCompression(const ZSTD_CCtx_params* cctxParam
         return 1;
     default:
         assert(0 /* impossible: pre-validated */);
-        /* fall-through */
+        ZSTD_FALLTHROUGH;
     case ZSTD_lcm_auto:
         return (cctxParams->cParams.strategy == ZSTD_fast) && (cctxParams->cParams.targetLength > 0);
     }

lib/compress/zstd_double_fast.c

@@ -244,8 +244,6 @@ _search_next_long:
             while (((ip>anchor) & (match>prefixLowest)) && (ip[-1] == match[-1])) { ip--; match--; mLength++; } /* catch up */
         }
 
-        /* fall-through */
-
 _match_found:
         offset_2 = offset_1;
         offset_1 = offset;