Do not set CMAKE_MATCH_ variables when not neeeded

Each call to AddDefinition has overhead for variable watches and such.
Avoid extra calls when not needed.

This decreases the configure time for ParaView by 10 seconds on my
machine.  Without the change about 1,000,000 set-to-empty calls were
being made.  After the change it drops to about 100,000.

Source/cmStringCommand.cxx

@@ -534,8 +534,12 @@ void cmStringCommand::ClearMatches(cmMakefile* mf)
     {
     char name[128];
     sprintf(name, "CMAKE_MATCH_%d", i);
-    mf->AddDefinition(name, "");
-    mf->MarkVariableAsUsed(name);
+    const char* s = mf->GetDefinition(name);
+    if(s && *s != 0)
+      {
+      mf->AddDefinition(name, "");
+      mf->MarkVariableAsUsed(name);
+      }
     }
 }
 
@@ -544,10 +548,14 @@ void cmStringCommand::StoreMatches(cmMakefile* mf,cmsys::RegularExpression& re)
 {
   for (unsigned int i=0; i<10; i++)
     {
-    char name[128];
-    sprintf(name, "CMAKE_MATCH_%d", i);
-    mf->AddDefinition(name, re.match(i).c_str());
-    mf->MarkVariableAsUsed(name);
+    std::string m = re.match(i);
+    if(m.size() > 0)
+      {
+      char name[128];
+      sprintf(name, "CMAKE_MATCH_%d", i);
+      mf->AddDefinition(name, re.match(i).c_str());
+      mf->MarkVariableAsUsed(name);
+      }
     }
 }