Apply suggestions from code review

Co-authored-by: Zachary Ware <zachary.ware@gmail.com>

Author: skirpichev

Doc/whatsnew/3.16.rst

@@ -173,6 +173,8 @@ Build changes
   to simplify maintenence and updates. The :mod:`decimal` module will now
   unconditionally use the system's libmpdec decimal library. Also remove the
   now unused :option:`!--with-system-libmpdec` :program:`configure` flag.
+  This change has no impact on binary releases of Python, which have been
+  built against a separate copy of libmpdec for the past several releases.
 
   (Contributed by Sergey B Kirpichev in :gh:`115119`.)
 

configure.ac

@@ -8280,10 +8280,6 @@ PY_STDLIB_MOD([_curses_panel],
   [], [test "$have_curses" = "yes" && test "$have_panel" = "yes"],
   [$PANEL_CFLAGS $CURSES_CFLAGS], [$PANEL_LIBS $CURSES_LIBS]
 )
-if test "$have_mpdec" = "no"
-then
-  AS_VAR_SET([py_cv_module_]_decimal, [missing])
-fi
 PY_STDLIB_MOD([_decimal],
   [], [test "$have_mpdec" = "yes"],
   [$LIBMPDEC_CFLAGS], [$LIBMPDEC_LIBS])