build: Break if ncurses is requested but unavailable

If building dselect is requested (the default), ncurses is requested
through DPKG_LIB_CURSES, but if it isn't found, only AC_MSG_WARN is
used, leading to a successful configure and a failure to build
dselect/*. Use AC_MSG_ERROR instead to make configure bail out.

Reported-by: Michael Schmidt <michael.schmidt.dangel@gmail.com>
Signed-off-by: Guillem Jover <guillem@debian.org>

m4/dpkg-libs.m4

@@ -96,7 +96,7 @@ AC_CHECK_LIB([ncurses], [initscr],
   [CURSES_LIBS="${CURSES_LIBS:+$CURSES_LIBS }-lncurses"],
   [AC_CHECK_LIB([curses], [initscr],
      [CURSES_LIBS="${CURSES_LIBS:+$CURSES_LIBS }-lcurses"],
-     [AC_MSG_WARN([no curses library found])])])])
+     [AC_MSG_ERROR([no curses library found])])])])
 ])# DPKG_CHECK_LIB_CURSES_NARROW
 
 # DPKG_LIB_CURSES