Fix webkit native to work on Ubuntu 16.04

I got an error that isnan was not defined. A simple qualifier std::isnan got me past the error. I have not tested this on other systems.

ddhill@ddhill-pad:~/sb/9/refbld/rt$ hg diff
diff --git a/modules/javafx.web/src/main/native/Source/JavaScriptCore/runtime/Options.cpp b/modules/javafx.web/src/main/native/Source/JavaScriptCore/runtime/Options.cpp
--- a/modules/javafx.web/src/main/native/Source/JavaScriptCore/runtime/Options.cpp
+++ b/modules/javafx.web/src/main/native/Source/JavaScriptCore/runtime/Options.cpp
@@ -506,7 +506,7 @@
     case Options::Type::unsignedType:
         return m_entry.unsignedVal == other.m_entry.unsignedVal;
     case Options::Type::doubleType:
- return (m_entry.doubleVal == other.m_entry.doubleVal) || (isnan(m_entry.doubleVal) && isnan(other.m_entry.doubleVal));
+ return (m_entry.doubleVal == other.m_entry.doubleVal) || (std::isnan(m_entry.doubleVal) && std::isnan(other.m_entry.doubleVal));
     case Options::Type::int32Type:
         return m_entry.int32Val == other.m_entry.int32Val;
     case Options::Type::optionRangeType: