diff -uNr passenger-5.1.1.orig/src/cxx_supportlib/vendor-modified/boost/math/special_functions/fpclassify.hpp passenger-5.1.1/src/cxx_supportlib/vendor-modified/boost/math/special_functions/fpclassify.hpp
--- passenger-5.1.1.orig/src/cxx_supportlib/vendor-modified/boost/math/special_functions/fpclassify.hpp	2013-10-26 15:00:00.000000000 -0700
+++ passenger-5.1.1/src/cxx_supportlib/vendor-modified/boost/math/special_functions/fpclassify.hpp	2017-01-12 19:21:39.696285758 -0800
@@ -134,8 +134,8 @@
 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); }
 #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)); }
+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)); }
 #endif
 #endif
 }