tinyexpr: use std:: namespace for older libstdc++

Fixes the build on Ubuntu Xenial 16.04 and CentOS 7.

src/tinyexpr.cpp

@@ -38,6 +38,12 @@
 #include <iterator>
 #include <utility>
 
+// Older version of libstdc++ (GCC 5 and before) need this as wutil.h includes common.h, which
+// includes <algorithm>, which includes <cmath> - meaning the functions get included into the std::
+// namespace, and including math.h later does not reimport them.
+using std::isnan;
+using std::signbit;
+
 // TODO: It would be nice not to rely on a typedef for this, especially one that can only do
 // functions with two args.
 using te_fun2 = double (*)(double, double);