Fully qualify isnan:
* /var/tmp/portage/sci-physics/geant-4.10.02/work/geant4.10.02/source/processes/hadronic/models/lend/src/nf_utilities.cc:61:22: error: ‘isnan’ was not declared in this scope
*      return( isnan( d ) );
See also:
https://bugs.gentoo.org/show_bug.cgi?id=581322

--- geant4.10.02/source/processes/hadronic/models/lend/src/nf_utilities.cc
+++ geant4.10.02/source/processes/hadronic/models/lend/src/nf_utilities.cc
@@ -58,7 +58,7 @@
 */
 int nfu_isNAN( double d ) {
 
-    return( isnan( d ) );
+    return( std::isnan( d ) );
 }
 /*
 ************************************************************