Main: omit "constexpr" on MIN_BUFFER_SIZE with GCC 4.x

NEWS

 ver 0.20.8 (not yet released)
 * output
   - osx: fix build failure due to missing "noexcept"
+* fix build failure with GCC 4.x
 
 ver 0.20.7 (2017/05/15)
 * database

src/Main.cxx

@@ -121,8 +121,16 @@ static constexpr size_t KILOBYTE = 1024;
 static constexpr size_t MEGABYTE = 1024 * KILOBYTE;
 
 static constexpr size_t DEFAULT_BUFFER_SIZE = 4 * MEGABYTE;
-static constexpr size_t MIN_BUFFER_SIZE = std::max(CHUNK_SIZE * 32,
-						   64 * KILOBYTE);
+
+static
+#if GCC_OLDER_THAN(5,0)
+/* gcc 4.x has no "constexpr" for std::max() */
+const
+#else
+constexpr
+#endif
+size_t MIN_BUFFER_SIZE = std::max(CHUNK_SIZE * 32,
+				  64 * KILOBYTE);
 
 static constexpr unsigned DEFAULT_BUFFER_BEFORE_PLAY = 10;