[v8] Fix compile failure when using gcc and libc++

R=machenback@chromium.org

Bug: v8:6565
Change-Id: I57bf7deb926e2b2b0623d8f2a65693df4639aaa2
Reviewed-on: https://chromium-review.googlesource.com/560092Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46423}

src/conversions.cc

@@ -20,9 +20,11 @@
 #include "src/strtod.h"
 #include "src/utils.h"
 
-#ifndef _STLP_VENDOR_CSTD
+#if defined(_STLP_VENDOR_CSTD)
 // STLPort doesn't import fpclassify into the std namespace.
-using std::fpclassify;
+#define FPCLASSIFY_NAMESPACE
+#else
+#define FPCLASSIFY_NAMESPACE std
 #endif
 
 namespace v8 {
@@ -122,7 +124,7 @@ double StringToInt(UnicodeCache* unicode_cache,
 
 
 const char* DoubleToCString(double v, Vector<char> buffer) {
-  switch (fpclassify(v)) {
+  switch (FPCLASSIFY_NAMESPACE::fpclassify(v)) {
     case FP_NAN: return "NaN";
     case FP_INFINITE: return (v < 0.0 ? "-Infinity" : "Infinity");
     case FP_ZERO: return "0";