Update build process for macOS.

Makefile

@@ -92,7 +92,7 @@ linux_ARCH_LIBS:=obj/postscript_print.o
 ARCH_LIBS:=$($(OS)_ARCH_LIBS)
 
 windows_ARCH_CFLAGS:=
-osx_ARCH_CFLAGS:=-isystem /opt/local/include -DHAVE_STRCASESTR -w -headerpad_max_install_names
+osx_ARCH_CFLAGS:=-mmacosx-version-min=10.7 -isystem /opt/local/include -DHAVE_STRCASESTR -w -headerpad_max_install_names
 beos_ARCH_CFLAGS:=
 linux_ARCH_CFLAGS:=
 ARCH_CFLAGS:=$($(OS)_ARCH_CFLAGS)

macos/Info.plist

@@ -23,6 +23,6 @@
 	<key>CFBundleSignature</key>
 	<string>TXPT</string>
 	<key>CFBundleVersion</key>
-	<string>2017-11-27</string>
+	<string>2017-12-24</string>
 </dict>
 </plist>

macos/README.txt

@@ -10,8 +10,11 @@ Linux application.
 REQUIREMENTS
 ------------
 Although Tux Paint is run without the XCode IDE, XCode itself is still required
-to build Tux Paint. Download it from the App Store, and launch it once to
+to build Tux Paint.  Download it from the App Store, and launch it once to
-accept its license agreements.
+accept its license agreements.  Also install XCode command line tools using the
+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
@@ -23,6 +26,7 @@ As of this writing, the required libraries are:
 
   cairo
   fribidi
+  lbzip2
   libpaper
   libpng
   librsvg
@@ -55,3 +59,63 @@ Simply, run:
 ... to create the TuxPaint.app application bundle that can be run in-place or
 copied to /Applications.  Zip it up for distribution.
 
+
+KNOWN BUGS
+----------
+On macOS 10.13 High Sierra:
+- The cursor appears with an invert-transparent background due to an issue
+  with SDL1 on macOS 10.13.  The following bug reports have been made:
+
+  MacPorts defect #55804 - https://trac.macports.org/ticket/55804
+  SDL bug #4076 - https://bugzilla.libsdl.org/show_bug.cgi?id=4076
+
+
+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, 10.7 Lion and later), one of the following must
+be done:
+
+  (A) Pass the flag -mmacosx-version-min=10.7 to the compiler.
+  (B) Or set the environment variable MACOSX_DEPLOYMENT_TARGET to 10.7
+
+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 10.7 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   10.7
+
+  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, 10.7 is the oldest version of macOS that can be
+targetted without errors when compiling the sources of MacPorts libraries
+required by Tux Paint.
+
+4th March 2018
+Mark K. Kim <mkkim214@gmail.com>