Xilinx ISE: make project fails if project file exists
in Xilinx ISE, if the project file already exists, the
make project target fails with:
ERROR:TclTasksC:project_087: Project file "my_project.xise" already exists. Use [project open "my_project.xise"] instead.
Obviously, we should check against this and either open the project and edit it, or delete it and start from scratch.