cmDefinitions: Use a hashmap for faster checks

The hash map is much faster at checking that the map won't have what
we're looking for so that we can just go to the parent scope instead.

Source/cmDefinitions.cxx

@@ -15,7 +15,8 @@
 cmDefinitions::Def cmDefinitions::NoDef;
 
 //----------------------------------------------------------------------------
-cmDefinitions::cmDefinitions(cmDefinitions* parent): Up(parent)
+cmDefinitions::cmDefinitions(cmDefinitions* parent)
+  : Up(parent)
 {
 }
 
@@ -35,7 +36,7 @@ cmDefinitions::GetInternal(const std::string& key) const
     {
     return i->second;
     }
-  else if(cmDefinitions* up = this->Up)
+  if(cmDefinitions* up = this->Up)
     {
     // Query the parent scope.
     return up->GetInternal(key);

Source/cmDefinitions.h

@@ -13,6 +13,9 @@
 #define cmDefinitions_h
 
 #include "cmStandardIncludes.h"
+#if defined(CMAKE_BUILD_WITH_CMAKE)
+#include "cmsys/hash_map.hxx"
+#endif
 
 /** \class cmDefinitions
  * \brief Store a scope of variable definitions for CMake language.
@@ -71,7 +74,11 @@ private:
   cmDefinitions* Up;
 
   // Local definitions, set or unset.
+#if defined(CMAKE_BUILD_WITH_CMAKE)
+  typedef cmsys::hash_map<std::string, Def> MapType;
+#else
   typedef std::map<std::string, Def> MapType;
+#endif
   MapType Map;
 
   // Internal query and update methods.