bug#36995: 27.0.50; configure enables HAVE_JPEG in spite of --without-X

[Ulrich Mueller <ulm@gentoo.org>]

configure --without-X will result in HAVE_JPEG being enabled, if the
jpeg library is detected on the system.

Unless I am missing something, the only use of the HAVE_JPEG macro
(and inclusion of jpeglib.h) is in image.c, which is only being compiled
if a window system is enabled (via WINDOW_SYSTEM_OBJ). So enabling jpeg
when X is disabled doesn't serve any purpose.

Long time ago, all image formats used to be disabled when configured
--without-X. Behaviour changed between Emacs versions 24.5 and 25.1.
AFAICS, this is due to the following commit, which (intentionally or
unintentionally?) dropped the test for a window system:

commit 5761a2ecb1a5178d2ea69a39725bdee368a754a5
Author: Paul Eggert <eggert@cs.ucla.edu>
Date:   Wed Apr 15 18:30:01 2015 -0700

    Port jpeg configuration to Solaris 10 with Sun C
    
    * configure.ac: Check for jpeglib 6b by trying to link it, instead
    of relying on cpp magic that has problems in practice.  Check for
    both jpeglib.h and jerror.h features.  Remove special case for
    mingw32, which should no longer be needed (and if it were needed,
    should now be addressable by hotwiring emacs_cv_jpeglib).
    Fixes: bug#20332


In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu)
 of 2019-08-10 built on themis
Repository revision: 2c5dd680180e978303662d3d32cc9aa5121b4d29
Repository branch: master
System Description: Gentoo/Linux

Configured using:
 'configure --prefix=/usr --build=x86_64-pc-linux-gnu
 --host=x86_64-pc-linux-gnu --mandir=/usr/share/man
 --infodir=/usr/share/info --datadir=/usr/share --sysconfdir=/etc
 --localstatedir=/var/lib --disable-silent-rules
 --docdir=/usr/share/doc/emacs-vcs-27.0.9999
 --htmldir=/usr/share/doc/emacs-vcs-27.0.9999/html --libdir=/usr/lib64
 --program-suffix=-emacs-27-vcs --includedir=/usr/include/emacs-27-vcs
 --infodir=/usr/share/info/emacs-27-vcs --localstatedir=/var
 --enable-locallisppath=/etc/emacs:/usr/share/emacs/site-lisp
 --without-compress-install --without-hesiod --without-pop
 --with-file-notification=no --disable-acl --without-dbus
 --without-modules --without-gameuser --without-libgmp --without-gpm
 --without-json --without-kerberos --without-kerberos5 --without-lcms2
 --without-xml2 --without-mailutils --without-selinux --without-gnutls
 --without-libsystemd --without-threads --without-wide-int
 --without-zlib --with-sound=no --without-x --without-ns
 'CFLAGS=-march=native -ggdb -O2 -pipe' CPPFLAGS= 'LDFLAGS=-Wl,-O1
 -Wl,--as-needed''

Configured features:
JPEG XIM PDUMPER