The CHECK_TERM_ENV macro returns !NonEmpty(term_env), i.e. true if
term_env **is** empty. Negating the return value again causes setupterm
to mailfunction.

diff -upr ncurses-6.2-20200906.orig/ncurses/tinfo/lib_setup.c ncurses-6.2-20200906/ncurses/tinfo/lib_setup.c
--- ncurses-6.2-20200906.orig/ncurses/tinfo/lib_setup.c	2020-09-07 11:28:05.041642395 +0200
+++ ncurses-6.2-20200906/ncurses/tinfo/lib_setup.c	2020-09-07 11:28:32.087989409 +0200
@@ -667,7 +667,7 @@ TINFO_SETUP_TERM(TERMINAL **tp,
 	if (!NonEmpty(tname))
 	    tname = "unknown";
 #else
-	if (!CHECK_TERM_ENV(tname, NO_TERMINAL)) {
+	if (CHECK_TERM_ENV(tname, NO_TERMINAL)) {
 	    T(("Failure with TERM=%s", tname));
 	    ret_error0(TGETENT_ERR, "TERM environment variable not set.\n");
 	}