Disable XML validation. Enabling it leads to a circular build dependency.
Index: gap-gapdoc-1.6.5/makedocrel.g
===================================================================
--- gap-gapdoc-1.6.5.orig/makedocrel.g
+++ gap-gapdoc-1.6.5/makedocrel.g
@@ -26,7 +26,7 @@ maintree := MakeGAPDocDoc("doc", "gapdoc
 "../lib/XMLParser.gd", "../lib/Make.g" ], "GAPDoc", relpath, "MathJax");
 
 # check validity
-ValidateGAPDoc([maintree.input, maintree.inputorigins]);
+#ValidateGAPDoc([maintree.input, maintree.inputorigins]);
 
 CopyHTMLStyleFiles("doc");
 
@@ -41,7 +41,7 @@ exampletree :=
       "MathJax");
 
 # check validity
-ValidateGAPDoc([exampletree.input, exampletree.inputorigins]);
+#ValidateGAPDoc([exampletree.input, exampletree.inputorigins]);
 
 CopyHTMLStyleFiles("example");
 
@@ -51,7 +51,7 @@ Print("\n========== converting small exa
             "MathJax");
 
 # check validity
-ValidateGAPDoc([3kp1tree.input, 3kp1tree.inputorigins]);
+#ValidateGAPDoc([3kp1tree.input, 3kp1tree.inputorigins]);
 
 CopyHTMLStyleFiles("3k+1");
 
