--- include/geos/platform.h.in
+++ include/geos/platform.h.in
@@ -97,8 +97,9 @@
    // It does leave a version in std.
 #  define ISNAN(x) (std::isnan(x))
 # elif defined(__sun) || defined(__sun__)
+#  define _XOPEN_SOURCE 600  // force iso/math_c99
 #  include <math.h>
-#  define ISNAN(x) (::isnan(x))
+#  define ISNAN(x) (std::isnan(x))
 # endif
 #endif