whitespace-mode will highlight long lines starting at 70 characters
instead of what fill-column is set to, if whitespace-line-column is set
to nil.

To reproduce:

1. Create the file test.cfg with this as the first line:

# -*- fill-column: 200 -*-

2. Run emacs -Q

3. Execute the following commands:

(setq whitespace-line-column nil)
(add-hook 'conf-mode-hook
          (lambda () (whitespace-mode)))

4. Open test.cfg and type more than 70 characters

After disabling whitespace-mode and enabling it again,
whitespace-line-column is set correctly to 200.

In GNU Emacs 26.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.8)
 of 2019-07-02 built on ventiloplattform
Windowing system distributor 'The X.Org Foundation', version
11.0.12004000 System Description:	Gentoo Base System release 2.6

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
test.cfg has auto save data; consider M-x recover-this-file
Making completion list...

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-26.2
 --htmldir=/usr/share/doc/emacs-26.2/html --libdir=/usr/lib64
 --program-suffix=-emacs-26 --includedir=/usr/include/emacs-26
 --infodir=/usr/share/info/emacs-26 --localstatedir=/var
 --enable-locallisppath=/etc/emacs:/usr/share/emacs/site-lisp
 --without-compress-install --without-hesiod --without-pop
 --with-file-notification=inotify --enable-acl --with-dbus
 --with-modules --without-gameuser --with-gpm --without-kerberos
 --without-kerberos5 --with-lcms2 --with-xml2 --without-mailutils
 --without-selinux --with-gnutls --without-libsystemd --with-threads
 --without-wide-int --with-zlib --with-sound=no --with-x --without-ns
 --without-gconf --without-gsettings --with-toolkit-scroll-bars
 --with-gif --with-jpeg --with-png --with-rsvg --with-tiff --with-xpm
 --with-imagemagick --with-xft --without-cairo --without-libotf
 --without-m17n-flt --with-x-toolkit=gtk3 --with-xwidgets
 'CFLAGS=-march=native -O2 -pipe' CPPFLAGS= 'LDFLAGS=-Wl,-O1
 -Wl,--as-needed''

Configured features:
XPM JPEG TIFF GIF PNG RSVG GPM DBUS GLIB NOTIFY ACL GNUTLS LIBXML2
FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS
XWIDGETS LCMS2

Important settings:
  value of $LC_CTYPE: de_DE.utf8
  value of $LC_TIME: en_DK.UTF-8
  value of $LANG: de_DE.utf8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Conf[Space]

Minor modes in effect:
  whitespace-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv dired dired-loaddefs format-spec rfc822 mml
easymenu mml-sec password-cache epa derived epg epg-config gnus-util
rmail rmail-loaddefs mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils disp-table whitespace conf-mode edmacro
kmacro cl-loaddefs cl-lib elec-pair time-date mule-util tooltip eldoc
electric uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win
x-win term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads dbusbind inotify lcms2 dynamic-setting
font-render-setting xwidget-internal move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 100238 7390)
 (symbols 48 20661 1)
 (miscs 40 61 129)
 (strings 32 29484 1113)
 (string-bytes 1 794338)
 (vectors 16 15267)
 (vector-slots 8 515682 9350)
 (floats 8 51 66)
 (intervals 56 351 0)
 (buffers 992 14))