build: let ./configure fail (not just warn) when curses.h is missing

This fixes https://savannah.gnu.org/bugs/?52930

.
Reported-by: Michail Tsiolkas <mtsio@cryptolab.net>

configure.ac

@@ -565,10 +565,10 @@ if eval "test x$CURSES_LIB_NAME = x"; then
 fi
 
 if eval "test x$CURSES_LIB_NAME = x"; then
-	AC_MSG_WARN([
-*** No curses lib available.  Consider getting the official ncurses
-*** distribution from ftp://ftp.gnu.org/pub/gnu/ncurses if you get
-*** errors compiling nano.])
+	AC_MSG_ERROR([
+*** No curses lib was found.  Please install the curses header files
+*** from libncursesw5-dev (Debian), ncurses-devel (Fedora), or similar.
+*** (Or install ncurses from https://ftp.gnu.org/gnu/ncurses/.)])
 else
 	AC_MSG_RESULT([Using $CURSES_LIB_NAME as the curses library])
 fi