Explicitly install CHANGES file, to avoid confusion and build failures when they aren't installed by default

Definitively avoid #920459, which accidentally recurred in one release.

... via override_dh_installchangelogs.

* Revert "documentation/make_pdf: Substitute xltabular for longtabu
  (#921294)." [commit 74a58584]
* debian/control: Add a versioned Build-Depends-Indep entry for
  texlive-latex-extra to avoid (long)tabu lossage from #920459.
  (Closes: #921294.)

See fltk1.1 bug #909078.

* configure.ac: Use (PKG_PROG_)PKG_CONFIG.
* debian/rules: Specify a host-specific pkg-config, both in FTCONFIG
  and in the environment.

... instead of calling dpkg-{architecture,buildflags} directly (and piecemeal).

The result doesn't look quite as good as before, but at least it builds
again with current TeX Live.  (Closes: #921294, fallout from #920621).

* debian/*-none*: Formally renamed from debian/*-noconfig* and tweaked
  correspondingly to account for running cmake via dh_auto_configure,
  which supplies -DCMAKE_BUILD_TYPE=None.
* debian/rules: Run cmake via dh_auto_configure per H. Grohne's suggestion;
  account for formal side effects of gaining -DCMAKE_BUILD_TYPE=None.

* configure: Revert local changes in favor of using dh_autoreconf.
* debian/rules: Allow (dh_)autoreconf after all, albeit with autoheader
  suppressed.

* debian/compat: Advance to 11.
* debian/control: Advance debhelper Build-Depends version constraint
  to (>= 11~).

* Set DEB_BUILD_MAINT_OPTIONS more readably.
* Use ?= for DEB_HOST_(MULTI)ARCH.

debian/rules: Pass "/usr/bin/pkg-config freetype2" off as FTCONFIG
(in lieu of freetype-config, which is now deprecated).

FLTK-Targets.cmake already declares it.

* debian/fix-fltk-targets-noconfig: Correct overzealous escaping that led
  to doubled multiarch paths for some settings.  (Closes: #871508.)

* debian/rules: Implement nodoc support, required as of Policy 4.0.0.
* Standards-Version: 4.0.1 (now compliant).

* debian/{compat,control}: Advance to Debhelper 10.
* debian/rules: Run dh --without autoreconf.  FLTK uses a
  hand-maintained configh.in and an autoheader-unfriendly configure.ac,
  breaking autoreconf (which runs by default as of compat 10).

Escape some left braces meant to be taken literally, for compatibility
with Perl 5.26.  (Closes: #871059.)

debian/fix-fltk-targets-noconfig: Append logic to refrain from
enforcing the existence of fluid in FLTK_SKIP_FLUID mode, based
on a suggestion from Gianfranco Costamagna <locutusofborg@d.o>.
(Closes: #855040.)

Thanks again to Javier Serrano Polo <javier@jasp.net>.

Thanks to Javier Serrano Polo <javier@jasp.net>.

Sourceful upload resynchronizes all architectures.  (Closes: #850924.)

[dgit (3.4) quilt-fixup]

In particular, declare Standards-Version: 3.9.8 (already compliant,
not that cme checked).

* debian/control: Retire explicit libfltk1.3-dbg package in favor of
  automatic -dbgsym packages.
* debian/rules: Stop collecting debug symbols in an explicit
  libfltk1.3-dbg package.
* debian/.gitignore: Drop libfltk1.3-dbg.

* debian/rules:
  - Drop the fltk-config.unhardened business, which proved insufficient.
    (Do, however, postprocess fltk-config to avoid emitting the
    -fdebug-prefix-map=... flag intended to *improve* reproducibility!)
  - Configure --with-optim=... so fltk-config --optim won't claim -Os.
* fltk-config.in: Filter out -D*, -f*, and -W*, not just -g* and -O* (but
  leave --optim output alone for reference.)
* .gitignore: Drop fltk-config.unhardened.