Fix building with C++14, which errors out due to bool -> T* conversions
See also: https://bugs.gentoo.org/show_bug.cgi?id=594376

--- a/src/SprBinaryEncoder.cc
+++ b/src/SprBinaryEncoder.cc
@@ -165,7 +165,7 @@
   if( found != vars.end() ) {
     cerr << "Variable " << sclass.c_str() << " is already included " 
 	 << "in the input list for SprBinaryEncoder." << endl;
-    return false;
+    return NULL;
   }
   vars.push_back(sclass);
 
--- a/src/SprDecisionTree.cc
+++ b/src/SprDecisionTree.cc
@@ -362,7 +362,7 @@
 {
   // header
   char s [200];
-  sprintf(s,"Trained DecisionTree %-6i signal nodes.    Overall FOM=%-10g W0=%-10g W1=%-10g N0=%-10i N1=%-10i    Version=%s",nodes1_.size(),fom_,w0_,w1_,n0_,n1_,SprVersion.c_str());
+  sprintf(s,"Trained DecisionTree %-6lu signal nodes.    Overall FOM=%-10g W0=%-10g W1=%-10g N0=%-10i N1=%-10i    Version=%s",nodes1_.size(),fom_,w0_,w1_,n0_,n1_,SprVersion.c_str());
   os << s << endl;
   os << "-------------------------------------------------------" << endl;
 
--- a/src/SprVarTransformerReader.cc
+++ b/src/SprVarTransformerReader.cc
@@ -50,7 +50,7 @@
   nLine++;
   if( !getline(is,line) ) {
     cerr << "Unable to read VarTransformer from line " << nLine << endl;
-    return 0;
+    return NULL;
   }
   istringstream ist(line);
   string dummy, transformerName, version;
@@ -59,9 +59,9 @@
   // decode name
   if( transformerName.empty() ) {
     cerr << "Unable to read VarTransformer name on line " << nLine << endl;
-    return false;
+    return NULL;
   }
-  SprAbsVarTransformer* t = 0;
+  SprAbsVarTransformer* t = NULL;
   if(      transformerName == "PCA" )
     t = SprVarTransformerReader::readPCATransformer(is,nLine);
   else if( transformerName == "InputNormalizer" )
@@ -72,16 +72,16 @@
     t = SprVarTransformerReader::readTransformerSequence(is,nLine);
   else {
     cerr << "Unknown VarTransformer name specified on line " << nLine << endl;
-    return 0;
+    return NULL;
   }
-  if( t == 0 ) return 0;
+  if( t == NULL ) return NULL;
   
   // read vars
   vector<string> oldVars, newVars;
   if( !SprVarTransformerReader::readVars(is,nLine,oldVars,newVars) ||
       oldVars.empty() || newVars.empty() ) {
     cerr << "Unable to read VarTransformer variables." << endl;
-    return 0;
+    return NULL;
   }
   t->setOldVars(oldVars);
   t->setNewVars(newVars);