Avoid linking with libform(w).so

If libtgent or libcurses is installed, the first one found of them
is linked to.

But if not, a find_package(Curses REQUIRED) is used and CURSES_LIBRARIES
is added to NVIM_LINK_LIBRARIES. This contains libform(w).so on many
systems, causing nvim to be linked to and depend on libform(w).so,
which may not be installed one some space-constrained systems, unnecessarily.
Merge #933 'Avoid linking with libform(w).so'