fix for Solaris Express 11 with CC 5.

git-svn-id: http://root.cern.ch/svn/root/trunk@41660 27541ba8-7e3a-0410-8455-c3a389f83636

math/mathcore/inc/TMath.h

@@ -503,7 +503,7 @@ inline Int_t TMath::Finite(Double_t x)
 
 inline Int_t TMath::IsNaN(Double_t x)
 #if defined(R__ANSISTREAM) || (defined(R__MACOSX) && defined(__arm__) )
-#ifdef isnan
+#if defined(isnan) || defined(R__SOLARIS_CC50)
    // from math.h
   { return ::isnan(x); }
 #else