bug#14921: 24.3.50; Disable mail menu items unless configured

[Drew Adams]

Please remove (not :visible) or disable (not :enable) all menu items
that have to do with mail or news unless Emacs is actually
configured for these, i.e., unless the menu items actually work.

In my case, on Windows, none of these are operational:

`Read Net News (Gnus)'
`Read Mail (with RMAIL)'
`Send Mail (with Gnus Message)'

It is impolite and disrespectul of users to pollute the menu space
with items and menus that are not usable.


In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2013-07-14 on ODIEONE
Bzr revision: 113423 lekktu@gmail.com-20130715004922-i67tg2ois14h3fpm
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/binary --enable-checking=yes,glyphs
 CFLAGS='-O0 -g3' CPPFLAGS='-Ic:/Devel/emacs/include'
 LDFLAGS='-Lc:/Devel/emacs/lib''