util/Compiler.h: add gcc_fallthrough

Works around build failures with ccache which may feed processed code
to GCC, which doesn't have the "fall through" code comments.

src/player/Thread.cxx

@@ -46,6 +46,7 @@
 #include "CrossFade.hxx"
 #include "tag/Tag.hxx"
 #include "Idle.hxx"
+#include "util/Compiler.h"
 #include "util/Domain.hxx"
 #include "thread/Name.hxx"
 #include "Log.hxx"
@@ -1171,6 +1172,7 @@ try {
 			}
 
 			/* fall through */
+			gcc_fallthrough;
 
 		case PlayerCommand::PAUSE:
 			next_song.reset();

src/util/Compiler.h

@@ -143,6 +143,12 @@
 #define gcc_flatten
 #endif
 
+#if CLANG_OR_GCC_VERSION(7,0)
+#define gcc_fallthrough __attribute__((fallthrough))
+#else
+#define gcc_fallthrough
+#endif
+
 #ifndef __cplusplus
 /* plain C99 has "restrict" */
 #define gcc_restrict restrict

src/util/format.c

@@ -19,6 +19,7 @@
  */
 
 #include "format.h"
+#include "util/Compiler.h"
 
 #include <stdbool.h>
 #include <stdio.h>
@@ -238,6 +239,7 @@ format_object2(const char *format, const char **last, const void *object,
 			}
 
 			/* fall through */
+			gcc_fallthrough;
 
 		default:
 			/* pass-through non-escaped portions of the format string */