cmGeneratorTarget: Move HasImportLibrary from cmTarget.

Source/cmGeneratorTarget.h

@@ -398,6 +398,9 @@ public:
   bool GetImplibGNUtoMS(std::string const& gnuName, std::string& out,
                         const char* newExt = 0) const;
 
+  /** Return whether or not the target has a DLL import library.  */
+  bool HasImportLibrary() const;
+
   struct SourceFileFlags
   GetTargetSourceFileFlags(const cmSourceFile* sf) const;