Allow features to be added to VF without building GDEF; warn when GDEF would be needed but isn't requested. This fixes #2694

Lib/fontTools/feaLib/builder.py

@@ -224,14 +224,14 @@ class Builder(object):
                 del self.font[tag]
         if any(tag in self.font for tag in ("GPOS", "GSUB")) and "OS/2" in self.font:
             self.font["OS/2"].usMaxContext = maxCtxFont(self.font)
+        gdef = self.buildGDEF()  # build unconditionally so we can warn below
         if "GDEF" in tables:
-            gdef = self.buildGDEF()
             if gdef:
                 self.font["GDEF"] = gdef
             elif "GDEF" in self.font:
                 del self.font["GDEF"]
-        elif self.varstorebuilder:
-            raise FeatureLibError("Must save GDEF when compiling a variable font", None)
+        elif gdef:
+            log.warning("GDEF is not requested, but is needed")
         if "BASE" in tables:
             base = self.buildBASE()
             if base: