build: make ./configure report which global nanorc file will be used

This addresses https://savannah.gnu.org/bugs/?57061.
Requested-by: Robert Johnson <aloha@blastpuppy.com>

Makefile.am

@@ -10,3 +10,9 @@ EXTRA_DIST = ChangeLog.1999-2006 ChangeLog.2007-2015 \
 		IMPROVEMENTS README.GIT
 
 ACLOCAL_AMFLAGS = -I m4
+
+showinfo:
+	@ echo
+	@ echo "  The global nanorc file is: @sysconfdir@/nanorc"
+	@ echo "  Syntaxes get installed in: @PKGDATADIR@/"
+	@ echo

configure.ac

@@ -596,7 +596,7 @@ if eval "test x$CURSES_LIB_NAME = x"; then
   *** 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])
+	AC_MSG_RESULT([  The curses library to be used is: $CURSES_LIB_NAME])
 fi
 
 AC_CHECK_LIB([$CURSES_LIB_NAME], [use_default_colors],
@@ -741,3 +741,5 @@ syntax/Makefile
 ])
 
 AC_OUTPUT
+
+make showinfo