Update readme with thirdparty links and Mac OS X instructions

README.md

@@ -29,8 +29,10 @@ need Microsoft Visual Studio to build Panda3D, though - the relevant compilers
 are included as part of the Windows 7.1 SDK.
 
 You will also need to have the third-party dependency libraries available for
-the build scripts to use.  These are available from here:
-https://www.panda3d.org/forums/viewtopic.php?f=9&t=16346
+the build scripts to use.  These are available from one of these two URLs,
+depending on whether you are on a 32-bit or 64-bit system:
+https://www.panda3d.org/download/panda3d-1.9.0/panda3d-1.9.0-tools-win32.zip
+https://www.panda3d.org/download/panda3d-1.9.0/panda3d-1.9.0-tools-win64.zip
 
 After acquiring these dependencies, you may simply build Panda3D from the
 command prompt using the following command:
@@ -81,3 +83,25 @@ your system.  Careful: it is not easy to uninstall Panda3D in this way!
 ```bash
 python2.7 makepanda/installpanda.py --prefix=/usr/local
 ```
+
+Mac OS X
+--------
+
+On Mac OS X, all you need to compile Panda3D is a set of precompiled
+thirdparty packages, which can be acquired from here:
+https://www.panda3d.org/download/panda3d-1.9.0/panda3d-1.9.0-tools-mac.tar.gz
+
+After placing the thirdparty directory inside the panda3d source directory,
+you may build Panda3D using a command like the following:
+
+```bash
+python makepanda/makepanda.py --everything --installer
+```
+
+In order to make a universal build, pass the --universal flag.  You may also
+target a specific minimum Mac OS X version using the --osxtarget flag followed
+by the release number, eg. 10.6 or 10.7.
+
+If the build was successful, makepanda will have generated a .dmg file in
+the source directory containing the installer.  Simply open it and run the
+package file in order to install the SDK onto your system.