Dear Aernout,
you can safely delete line 61 from the makefile (or use gnu make). The next release of xotcl will have this line from the makefile dropped.
Concerning the missing tclInt.h file: xotcl needs tclInt.h for compilation. In some installations, tclInt.h is not installed globally. Most probably, you got already an error message during configure. you should run configure with --with-tcl or --with-tclinclude pointing to the place where the tcl include files are located.
