editors/texmacs: try to unbreak the build against modern C++ compilers

Add parentheses around `*this' so that hashtree<K,V>::operator-> kicks
in which returns a pointer to hashtree_rep<K,V> as expected (note that
similar hashtree<K,V>::operator() method below does not have this bug).

Chase HTTP/1.1 301 redirection in the MASTER_SITES while here and drop
no longer needed GNU_CONFIGURE_MANPREFIX override.

Via: https://build.opensuse.org/package/show/openSUSE:Factory/TeXmacs