cmCacheManager: Return a C string from GetValue.

This is for consistency with other CMake access interfaces such as
definitions and properties which use a null value as a 'not present'
value.  It is source compatible with existing callers, and it returns
a pointer into actual real cache entry storage.

Source/cmCacheManager.h

@@ -65,7 +65,7 @@ public:
     void AppendProperty(const std::string& property, const char* value,
                         bool asString=false);
     void SetProperty(const std::string& property, bool value);
-    std::string GetValue() const { return this->GetEntry().Value; }
+    const char* GetValue() const { return this->GetEntry().Value.c_str(); }
     bool GetValueAsBool() const;
     void SetValue(const char*);
     CacheEntryType GetType() const { return this->GetEntry().Type; }