[osx] Fix "'xcodebuild' requires Xcode" error

By default, the `xcode-select` developer directory points to
`/Library/Developer/CommandLineTools`, but since it doesn't
get (immediately?) updated to point to the appropriate directory
from within `Xcode.app` once `Xcode` is installed, the following
error will be raised when trying to install anything that require
the `Xcode` CLTs:

 " xcode-select: error: tool 'xcodebuild' requires Xcode, but
   active developer directory '/Library/Developer/CommandLineTools'
   is a command line tools instance. "

This commit fixes the issue described above by making the
`xcode-select` developer directory point to the appropriate
directory from within `Xcode.app` once Xcode is installed.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Fix #13
Ref #13

os/os_x/installs/install_xcode.sh

@@ -13,6 +13,8 @@ main() {
         # Prompt user to install the XCode Command Line Tools
         xcode-select --install &> /dev/null
 
+        # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
         # Wait until the XCode Command Line Tools are installed
         while ! xcode-select -p &> /dev/null; do
             sleep 5
@@ -22,6 +24,15 @@ main() {
 
         # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
+        # Point the `xcode-select` developer directory to
+        # the appropriate directory from within `Xcode.app`
+        # https://github.com/alrra/dotfiles/issues/13
+
+        sudo xcode-select -switch /Applications/Xcode.app/Contents/Developer
+        print_result $? 'Make "xcode-select" developer directory point to Xcode'
+
+        # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
         # Prompt user to agree to the terms of the Xcode license
         # https://github.com/alrra/dotfiles/issues/10