Fix warning for g++ 4.9.3.

Changelog

diff --git a/Changelog b/Changelog
index 95a288a..647a0d8 100644
--- a/Changelog
+++ b/Changelog
@@ -1,3 +1,7 @@
+2019-04-22:
+  Fixed warning in gcc4.9. Thanks to Scott McCaskill
+    (https://github.com/usefulcat) for the patch.
+
 2019-04-16:
   Merged changes to install libraries in the correct place when
   using 64-bit libraries.

double-conversion/utils.h

diff --git a/double-conversion/utils.h b/double-conversion/utils.h
index 7c10d3f..4328344 100644
--- a/double-conversion/utils.h
+++ b/double-conversion/utils.h
@@ -56,6 +56,13 @@
 #endif
 #endif
 
+#ifndef DOUBLE_CONVERSION_UNUSED
+#ifdef __GNUC__
+#define DOUBLE_CONVERSION_UNUSED __attribute__((unused))
+#else
+#define DOUBLE_CONVERSION_UNUSED
+#endif
+#endif
 
 // Double operations detection based on target architecture.
 // Linux uses a 80bit wide floating point stack on x86. This induces double
@@ -341,6 +348,7 @@
   static_assert(sizeof(Dest) == sizeof(Source),
                 "source and destination size mismatch");
 #else
+  DOUBLE_CONVERSION_UNUSED
   typedef char VerifySizesAreEqual[sizeof(Dest) == sizeof(Source) ? 1 : -1];
 #endif