GNUstep make and NEEDS_GUI

By default, GNUstep make encodes assumptions about whether it should link your target to the GUI framework (libs-gui on most platforms, AppKit on macOS). The default is yes for apps, and no for frameworks, libraries, and tools.

To change this, modify your GNUmakefile to set one of these variables to either yes or no:

• NEEDS_GUI: applies to all targets in the GNUmakefile.
• [product_name]_NEEDS_GUI: applies to the specific product. For example, if your tool is called MyTool, and it needs to link to the GUI framework, set MyTool_NEEDS_GUI = yes.