Add fallthrough attribute to suppress GCC 7 warnings

GCC 7 warns when fallthrough occurs in a switch statement, unless a
fallthrough attribute is defined. Add this fallthrough element, hidden
behind a macro.

lib/arch/generic/dec_tail.c

@@ -10,6 +10,9 @@
 		st.carry = q << 2;
 		st.bytes++;
 
+		// Deliberate fallthrough:
+		BASE64_FALLTHROUGH
+
 	case 1:	if (srclen-- == 0) {
 			ret = 1;
 			break;
@@ -24,6 +27,9 @@
 		st.bytes++;
 		outl++;
 
+		// Deliberate fallthrough:
+		BASE64_FALLTHROUGH
+
 	case 2:	if (srclen-- == 0) {
 			ret = 1;
 			break;
@@ -56,6 +62,9 @@
 		st.bytes++;
 		outl++;
 
+		// Deliberate fallthrough:
+		BASE64_FALLTHROUGH
+
 	case 3:	if (srclen-- == 0) {
 			ret = 1;
 			break;

lib/arch/generic/enc_tail.c

@@ -6,6 +6,9 @@
 		st.bytes++;
 		outl += 1;
 
+		// Deliberate fallthrough:
+		BASE64_FALLTHROUGH
+
 	case 1:	if (srclen-- == 0) {
 			break;
 		}
@@ -14,6 +17,9 @@
 		st.bytes++;
 		outl += 1;
 
+		// Deliberate fallthrough:
+		BASE64_FALLTHROUGH
+
 	case 2:	if (srclen-- == 0) {
 			break;
 		}

lib/codecs.h

@@ -110,8 +110,16 @@ struct codec
 // End-of-file when stream end has been reached or invalid input provided:
 #define BASE64_EOF 2
 
+// GCC 7 defaults to issuing a warning for fallthrough in switch statements,
+// unless the fallthrough cases are marked with an attribute. As we use
+// fallthrough deliberately, define an alias for the attribute:
+#if __GNUC__ >= 7
+  #define BASE64_FALLTHROUGH  __attribute__((fallthrough));
+#else
+  #define BASE64_FALLTHROUGH
+#endif
 
-void codec_choose (struct codec *, int flags);
+extern void codec_choose (struct codec *, int flags);
 
 // These tables are used by all codecs
 // for fallback plain encoding/decoding: