mx_termcap_init(): fix: no auto_right_margin except with library support..
In
79fee7e46da86fa0a1829fc09849070f2cae6e03 (termcap care for
auto_right_margin/eat_newline_glitch (Jörg Schilling)..,
2018-11-19) we started looking out for "!am || xenl".
However, when build without library support this effectively
always sets free the full terminal width since *termcap* cannot
"not include" a boolean! Therefore stop using "am" unless we do
have full terminal support, and only lookup xenl
eat_newline_glitch otherwise.