Update install notes with instructions for building using Xcode

Notable document the special incantation needed by Xcode to use the
inherited environment variables.

Closes https://github.com/wxWidgets/wxWidgets/pull/2603

docs/doxygen/overviews/install.md

@@ -190,6 +190,36 @@ For applications using Microsoft Visual Studio IDE, simply add the provided
 [instructions](@ref msw_build_apps) and build the project as usual.
 
 
+### Mac, with Xcode
+
+If you want to use an environment variable (such as WXWIN) in your xcode
+project, there are several things you must do.
+
+- Enable the usage of environment variables in xcode:<br>
+  `defaults write com.apple.dt.Xcode UseSanitizedBuildSystemEnvironment -bool NO`
+- Set the variables for use with the launch agent (application to OSX 10.10
+and up)
+```
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+  <key>Label</key>
+  <string>my.startup</string>
+  <key>ProgramArguments</key>
+  <array>
+    <string>sh</string>
+    <string>-c</string>
+    <string>
+launchctl setenv WXWIN /Users/dconnet/devtools/wx/wxWidgets-3.1.5
+    </string>
+  </array>
+  <key>RunAtLoad</key>
+  <true/>
+</dict>
+</plist>
+```
+
 ### Other IDEs
 
 If you use an IDE with wxWidgets support, such as [Code::Blocks][1] or