Fix warnings on Win64.

src/flags.cc

Index: src/flags.cc
diff --git a/src/flags.cc b/src/flags.cc
index 5df3afd5f967ab23cae79ce9b06c0d76b8ccf68e..d444c976fbbc9c6ca723ccf06cb28cb1401a1644 100644
--- a/src/flags.cc
+++ b/src/flags.cc
@@ -303,8 +303,8 @@ static void SplitArgument(const char* arg,
     // get the value if any
     if (*arg == '=') {
       // make a copy so we can NUL-terminate flag name
-      int n = arg - *name;
-      CHECK(n < buffer_size);  // buffer is too small
+      size_t n = arg - *name;
+      CHECK(n < static_cast<size_t>(buffer_size));  // buffer is too small
       memcpy(buffer, *name, n);
       buffer[n] = '\0';
       *name = buffer;