bug#18650: 25.0.50; Error message 'zlib library not found'

[Dani Moncayo]

Recipe:
* If you have 'zlib1.dll' on path, rename it temporarily to something
  else.
* emacs -Q
* C-x C-f M-n

I see an annoying error message in the echo area: 'zlib library not
found'.

IIUC, Emacs should not show such message.  Instead, if it doesn't find
an optional library at runtime, it should just proceed without using the
feature(s) provided by the library.

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-06 on LEG570
Repository revision: 118063
monnier@iro.umontreal.ca-20141006174756-y7ha091r491l1ijw
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs 'CFLAGS=-O0 -g3'
 CPPFLAGS=-DGLYPH_DEBUG=1'

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY ACL GNUTLS LIBXML2 ZLIB

Important settings:
  value of $LANG: ESN
  locale-coding-system: cp1252


-- 
Dani Moncayo