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.

defs.h

@@ -27,6 +27,26 @@ uintmax_t strtoumax(const char *nptr, char **endptr, int base);
 #include <inttypes.h>
 #endif
 
+#if defined __GNUC__ || defined __clang__
+/*
+ * On MinGW, the correct compiler format checking for vsnprintf() etc
+ * can depend on compile-time flags; these control whether you get
+ * ISO C or Microsoft's non-standard format strings.
+ * We sometimes use __attribute__ ((format)) for our own printf-like
+ * functions, which are ultimately interpreted by the toolchain-chosen
+ * printf, so we need to take that into account to get correct warnings.
+ */
+#ifdef __MINGW_PRINTF_FORMAT
+#define PRINTF_LIKE(fmt_index, ellipsis_index) \
+    __attribute__ ((format (__MINGW_PRINTF_FORMAT, fmt_index, ellipsis_index)))
+#else
+#define PRINTF_LIKE(fmt_index, ellipsis_index) \
+    __attribute__ ((format (printf, fmt_index, ellipsis_index)))
+#endif
+#else /* __GNUC__ */
+#define PRINTF_LIKE(fmt_index, ellipsis_index)
+#endif /* __GNUC__ */
+
 typedef struct conf_tag Conf;
 typedef struct terminal_tag Terminal;
 typedef struct term_utf8_decode term_utf8_decode;