Greatly improve printf format-string checking.

I've added the gcc-style attribute("printf") to a lot of printf-shaped
functions in this code base that didn't have it. To make that easier,
I moved the wrapping macro into defs.h, and also enabled it if we
detect the __clang__ macro as well as __GNU__ (hence, it will be used
when building for Windows using clang-cl).

The result is that a great many format strings in the code are now
checked by the compiler, where they were previously not. This causes
build failures, which I'll fix in the next commit.

pageant.c

@@ -308,8 +308,9 @@ void pageant_unregister_client(PageantClient *pc)
     sfree(pc->info);
 }
 
-static void failure(PageantClient *pc, PageantClientRequestId *reqid,
-                    strbuf *sb, const char *fmt, ...)
+static PRINTF_LIKE(4, 5) void failure(PageantClient *pc,
+                                      PageantClientRequestId *reqid,
+                                      strbuf *sb, const char *fmt, ...)
 {
     strbuf_clear(sb);
     put_byte(sb, SSH_AGENT_FAILURE);