Fixed compilation error due to missing functions from cmath

IBM XLC has no functions from <cmath> under the std namespace. Added
a workaround for XLC on AIX.

Foundation/include/Poco/FPEnvironment_C99.h

@@ -25,6 +25,22 @@
 #include <cmath>
 
 
+#if defined(POCO_OS_AIX)
+
+
+namespace std {
+
+
+using ::isinf;
+using ::isnan;
+
+
+} // namespace std
+
+
+#endif
+
+
 namespace Poco {