Add API to extract target names from a genex string.

The TARGET_NAME expression, which requires a literal, provides
target names. $<TARGET_PROPERTY:tgt,prop> also provides target
names in the cases where tgt is a literal, so that TARGET_NAME is
not needed then in addition.

Source/cmExportFileGenerator.h

@@ -94,6 +94,10 @@ protected:
                                    cmTarget* depender,
                                    cmTarget* dependee) = 0;
 
+  void ResolveTargetsInGeneratorExpressions(std::string &input,
+                                    cmTarget* target,
+                                    std::vector<std::string> &missingTargets);
+
   // The namespace in which the exports are placed in the generated file.
   std::string Namespace;