requirements: externalise apr requirement

Removes the apr requirement from this internal document in favour
of making it an external requirement, which allows us to use an
apr formula to provide apr instead of asking people to install the
whole CLT package.

Library/Homebrew/requirements.rb

@@ -1,4 +1,5 @@
 require 'requirement'
+require 'requirements/apr_dependency'
 require 'requirements/fortran_dependency'
 require 'requirements/language_module_dependency'
 require 'requirements/minimum_macos_requirement'
@@ -136,27 +137,3 @@ class JavaDependency < Requirement
     s
   end
 end
-
-class AprDependency < Requirement
-  fatal true
-
-  satisfy(:build_env => false) { MacOS::CLT.installed? }
-
-  def message
-    message = <<-EOS.undent
-      Due to packaging problems on Apple's part, software that compiles
-      against APR requires the standalone Command Line Tools.
-    EOS
-    if MacOS.version >= :mavericks
-      message += <<-EOS.undent
-        Run `xcode-select --install` to install them.
-      EOS
-    else
-      message += <<-EOS.undent
-        The standalone package can be obtained from
-        https://developer.apple.com/downloads/,
-        or it can be installed via Xcode's preferences.
-      EOS
-    end
-  end
-end