To link the external files located outside the project directory, to the PSoC Designer project, Local.mk file has to be edited. This is explained in "IDE User Guide" Documentation. ( Help -> Documentation -> Designer Specific Documents) in section B.5.3.
The Sample project has been attached here which has been compiled correctly in PD5.1 with Imagecraft compiler. By adding following lines to local.mk file, an external c file has been linked. To open local.mk file, click on Project -> "Open Local.mk for this Project". Add the following lines to it.