Fix printf formats

src/base/compiler-specific.h

Index: src/base/compiler-specific.h
diff --git a/src/base/compiler-specific.h b/src/base/compiler-specific.h
index ffd5a44e141d069fd8f11996ce15da2c41d02b6c..822893ffecf073924c03cfcef70760fc4f0cdde6 100644
--- a/src/base/compiler-specific.h
+++ b/src/base/compiler-specific.h
@@ -26,6 +26,17 @@
 #define WARN_UNUSED_RESULT /* NOT SUPPORTED */
 #endif
 
+// Tell the compiler a function is using a printf-style format string.
+// |format_param| is the one-based index of the format string parameter;
+// |dots_param| is the one-based index of the "..." parameter.
+// For v*printf functions (which take a va_list), pass 0 for dots_param.
+// (This is undocumented but matches what the system C headers do.)
+#if defined(__GNUC__)
+#define PRINTF_FORMAT(format_param, dots_param) \
+  __attribute__((format(printf, format_param, dots_param)))
+#else
+#define PRINTF_FORMAT(format_param, dots_param)
+#endif
 
 // The C++ standard requires that static const members have an out-of-class
 // definition (in a single compilation unit), but MSVC chokes on this (when