libbacktrace: use __attribute__((__fallthrough__))

Use an attribute rather than a comment when falling through a switch case.

	* internal.h (ATTRIBUTE_FALLTHROUGH): Define.
	* elf.c (elf_zlib_inflate): Use ATTRIBUTE_FALLTHROUGH.

elf.c

@@ -2081,10 +2081,10 @@ elf_zlib_inflate (const unsigned char *pin, size_t sin, uint16_t *zdebug_table,
 			{
 			case 6:
 			  *plen++ = prev;
-			  /* fallthrough */
+			  ATTRIBUTE_FALLTHROUGH;
 			case 5:
 			  *plen++ = prev;
-			  /* fallthrough */
+			  ATTRIBUTE_FALLTHROUGH;
 			case 4:
 			  *plen++ = prev;
 			}
@@ -2115,22 +2115,22 @@ elf_zlib_inflate (const unsigned char *pin, size_t sin, uint16_t *zdebug_table,
 			{
 			case 10:
 			  *plen++ = 0;
-			  /* fallthrough */
+			  ATTRIBUTE_FALLTHROUGH;
 			case 9:
 			  *plen++ = 0;
-			  /* fallthrough */
+			  ATTRIBUTE_FALLTHROUGH;
 			case 8:
 			  *plen++ = 0;
-			  /* fallthrough */
+			  ATTRIBUTE_FALLTHROUGH;
 			case 7:
 			  *plen++ = 0;
-			  /* fallthrough */
+			  ATTRIBUTE_FALLTHROUGH;
 			case 6:
 			  *plen++ = 0;
-			  /* fallthrough */
+			  ATTRIBUTE_FALLTHROUGH;
 			case 5:
 			  *plen++ = 0;
-			  /* fallthrough */
+			  ATTRIBUTE_FALLTHROUGH;
 			case 4:
 			  *plen++ = 0;
 			}

internal.h

@@ -56,6 +56,14 @@ POSSIBILITY OF SUCH DAMAGE.  */
 # endif
 #endif
 
+#ifndef ATTRIBUTE_FALLTHROUGH
+# if (GCC_VERSION >= 7000)
+#  define ATTRIBUTE_FALLTHROUGH __attribute__ ((__fallthrough__))
+# else
+#  define ATTRIBUTE_FALLTHROUGH
+# endif
+#endif
+
 #ifndef HAVE_SYNC_FUNCTIONS
 
 /* Define out the sync functions.  These should never be called if