Update macOS build instructions

- Updated to include instructions on how to build libimagequant from
  source.
- Removed backward compatibility build instructions (it's no longer
  possible)

macos/README.txt

@@ -16,17 +16,19 @@ command:
 
   xcode-select --install
 
-Building Tux Paint also requires various libraries from MacPorts.  Install them
-to the default /opt/local path according to the instructions found on their
-website:
+Building Tux Paint also requires various packages.  We install them from
+MacPorts where possible, source code otherwise.  Install MacPorts to the
+default /opt/local path according to the instructions found on their website:
 
   https://www.macports.org/
 
-As of this writing, the required libraries are:
+As of this writing, the following packages need to be installed:
 
+  ImageMagick
   cairo
   fribidi
   lbzip2
+  libimagequant*
   libpaper
   libpng
   librsvg
@@ -35,16 +37,26 @@ As of this writing, the required libraries are:
   libsdl_mixer
   libsdl_pango
   libsdl_ttf
+  pkgconfig
   zlib
 
-... and other non-library packages required by the Makefile are:
+  * - Not available from MacPorts as of this writing, see below
 
-  ImageMagick
-  pkgconfig
-
-... but you should intall any package that is required by the latest version of
+... but you should install any package that is required by the latest version of
 Tux Paint.
 
+  libimagequant
+  -------------
+  libimagequant is not available from MacPorts as of this writing.  It can be
+  installed from the source code as follows.  It should be installed to
+  /opt/local (same as MacPorts) for the library to be included in TuxPaint.dmg.
+
+    $ git clone https://github.com/ImageOptim/libimagequant.git
+    $ cd libimagequant
+    $ ./configure --prefix=/opt/local
+    $ make
+    $ sudo make install
+
 
   *** WARNING ***
   ---------------
@@ -65,65 +77,14 @@ Simply, run:
 copied to /Applications.  It also creates TuxPaint.dmg for distribution.
 
 
-KNOWN BUGS
-----------
-- It is no longer possible to build Tux Paint that is backward compatible with
-  older versions of macOS than the one on which it is built using the procedure
-  described in the below section, "BACKWARD COMPATIBILITY".  This limitation
-  appears to stem from some libraries required by Tux Paint now being built
-  using Rust that is unable to produce libraries that are backward compatible
-  with older versions of macOS.  The steps of the procedure are left intact
-  below anyway in case the situation changes in the near future, with the
-  exception that what once-referenced 10.7 now reference 10.8, believed to be
-  the oldest version of macOS that Tux Paint could be backward compatible with*
-  once the Rust issue has been addressed.
+KNOWN ISSUES
+------------
+- A macOS binary built on a specific version of macOS only runs on that version
+  of macOS or later.  To ensure Tux Paint can run on the oldest version of
+  macOS possible, build it on the oldest version of macOS available.  As of
+  this writing we know Tux Paint cannot be built to run on macOS 10.7 or
+  earlier.
 
 
-BACKWARD COMPATIBILITY
-----------------------
-Broadly speaking, a Mac binary built on macOS 10.12 Sierra (for example) runs
-only on macOS 10.12 and later.  To compile a binary that can also execute on an
-earlier version of macOS (say, 10.8 Mountain Lion and later), one of the
-following must be done:
-
-  (A) Pass the flag -mmacosx-version-min=10.8 to the compiler.
-  (B) Or set the environment variable MACOSX_DEPLOYMENT_TARGET to 10.8
-
-Tux Paint binary itself is built by doing (A) in the Makefile (by passing the
-parameter to osx_ARCH_CFLAGS).  However, the MacPorts libraries used by Tux
-Paint are not, so Tux Paint package itself will not run on any version of macOS
-earlier than the macOS on which it is built.
-
-To build the Tux Paint package that can run on earlier versions of macOS, the
-MacPorts libraries also need to be built with either #1 or #2.  This is done by
-configuring MacPorts to install all packages from their sources and build them
-to run on macOS 10.8 and later:
-
-  1. Install the MacPorts base normally.
-  2. Before installing any MacPorts package, add the following settings to
-     /opt/local/etc/macports/macports.conf:
-
-     buildfromsource            always
-     macosx_deployment_target   10.8
-
-  3. Install all packages normally.
-
-... then build Tux Paint normally.
-
-Please note building MacPorts packages from the source takes significantly
-longer than installing the prebuilt packages.
-
-If you have already installed MacPorts' prebuilt packages, it is possible to
-uninstall them, add the above configuration from step #2, then install them
-again from the source.  The MacPorts webpage on migration explains how uninstall
-packages and reinstall them after making system changes:
-
-  https://trac.macports.org/wiki/Migration
-
-Even when Tux Paint and MacPorts are built to run on a specific version of
-macOS, it is possible Tux Paint will not compile for or run on that version of
-macOS.  As of this writing, 10.8 is the oldest version of macOS that can be
-targetted without errors when compiling the sources of MacPorts libraries
-required by Tux Paint.
-
+August 2, 2020
 Mark K. Kim <markuskimius@gmail.com>