File fix-undeclared-isnan.patch of Package rubygem-passenger

--- a/src/cxx_supportlib/vendor-modified/boost/math/special_functions/fpclassify.hpp
+++ b/src/cxx_supportlib/vendor-modified/boost/math/special_functions/fpclassify.hpp
@@ -133,6 +133,10 @@ inline bool is_nan_helper(T, const boost::false_type&)
 #if defined(BOOST_MATH_HAS_QUADMATH_H)
 inline bool is_nan_helper(__float128 f, const boost::true_type&) { return ::isnanq(f); }
 inline bool is_nan_helper(__float128 f, const boost::false_type&) { return ::isnanq(f); }
+#elif defined(BOOST_GNU_STDLIB) && BOOST_GNU_STDLIB && \
+      _GLIBCXX_USE_C99_MATH && !_GLIBCXX_USE_C99_FP_MACROS_DYNAMIC
+inline bool is_nan_helper(__float128 f, const boost::true_type&) { return std::isnan(static_cast<double>(f)); }
+inline bool is_nan_helper(__float128 f, const boost::false_type&) { return std::isnan(static_cast<double>(f)); }
 #else
 inline bool is_nan_helper(__float128 f, const boost::true_type&) { return ::isnan(static_cast<double>(f)); }
 inline bool is_nan_helper(__float128 f, const boost::false_type&) { return ::isnan(static_cast<double>(f)); }