mac build: use old xcode to build qr scanner on El Capitan

contrib/osx/README.md

@@ -3,22 +3,40 @@ Building Mac OS binaries
 
 This guide explains how to build Electrum binaries for macOS systems.
 
-The build process consists of two steps:
 
 ## 1. Building the binary
 
-This needs to be done on a system running macOS or OS X. We use El Capitan (10.11.6) as building it on High Sierra
+This needs to be done on a system running macOS or OS X. We use El Capitan (10.11.6) as building it
+on High Sierra (or later)
 makes the binaries incompatible with older versions. 
 
-Before starting, make sure that the Xcode command line tools are installed (e.g. you have `git`).
 
+#### 1.1 Get Xcode
+
+Building the QR code reader (CalinsQRReader) requires full Xcode (not just command line tools).
+
+The last Xcode version compatible with El Capitan is Xcode 8.2.1
+
+Get it from [here](https://developer.apple.com/download/more/).
+
+Unfortunately, you need an "Apple ID" account.
+
+After downloading, uncompress it.
+
+Make sure it is the "selected" xcode (e.g.):
+
+    sudo xcode-select -s $HOME/Downloads/Xcode.app/Contents/Developer/
+
+
+#### 1.2 Build Electrum
 
     cd electrum
     ./contrib/osx/make_osx
     
-This creates a folder named Electrum.app.
+This creates both a folder named Electrum.app and the .dmg file.
 
-## 2. Building the image 
+
+## 2. Building the image deterministically (WIP)
 The usual way to distribute macOS applications is to use image files containing the 
 application. Although these images can be created on a Mac with the built-in `hdiutil`,
 they are not deterministic.
@@ -33,4 +51,4 @@ Copy the Electrum.app directory over and install the dependencies, e.g.:
 Then you can just invoke `package.sh` with the path to the app:
 
     cd electrum
-    ./contrib/osx/package.sh ~/Electrum.app/
+    ./contrib/osx/package.sh ~/Electrum.app/