Michael P. Reilly suggested this fix: makesetup wants to prepend
"$(srcdir)/" before all source files even when is starts with "/".
diff --git a/Modules/makesetup b/Modules/makesetup
index fb480dd..b29938c 100755
--- a/Modules/makesetup
+++ b/Modules/makesetup
@@ -179,6 +179,7 @@
 			objs="$objs $obj"
 			case $src in
 			glmodule.c) ;;
+			/*) ;;
 			*) src='$(srcdir)/'$src;;
 			esac
 			case $doconfig in