Add :ld64 dependency

This allows formulae which won't build with Tiger's ld to conditionally
request a dependency on the ld64 formula. This modifies the build
environment appropriately, and will only be active on Tiger.

Library/Homebrew/dependency_collector.rb

@@ -2,6 +2,7 @@ require 'dependency'
 require 'dependencies'
 require 'requirement'
 require 'requirements'
+require 'requirements/ld64_dependency'
 require 'set'
 
 ## A dependency is a formula that another formula needs to install.
@@ -95,6 +96,8 @@ class DependencyCollector
     when :python     then PythonInstalled.new(tags)
     when :python2    then PythonInstalled.new("2", tags)
     when :python3    then PythonInstalled.new("3", tags)
+    # Tiger's ld is too old to properly link some software
+    when :ld64       then LD64Dependency.new if MacOS.version < :leopard
     else
       raise "Unsupported special dependency #{spec}"
     end

Library/Homebrew/extend/ENV.rb

@@ -375,6 +375,13 @@ module HomebrewEnvExtension
       Hardware.processor_count
     end
   end
+
+  # ld64 is a newer linker provided for Xcode 2.5
+  def ld64
+    ld64 = Formula.factory('ld64')
+    self['LD'] = ld64.bin/'ld'
+    append "LDFLAGS", "-B#{ld64.bin.to_s+"/"}"
+  end
 end
 
 class << ENV

Library/Homebrew/requirements/ld64_dependency.rb

@@ -0,0 +1,11 @@
+require 'dependency'
+
+# This special dependency ensures that the Tigerbrew ld64
+# formula is used as gcc's ld in place of the old version
+# that comes with the OS.
+class LD64Dependency < Dependency
+  def initialize(name='ld64', tags=[:build])
+    @env_proc = proc { ENV.ld64 }
+    super
+  end
+end

Library/Homebrew/test/test_dependency_collector.rb

@@ -111,6 +111,16 @@ class DependencyCollectorTests < Test::Unit::TestCase
     assert_equal X11Dependency::Proxy.new(:libpng), @d.build(:libpng)
   end
 
+  def test_ld64_dep_pre_leopard
+    MacOS.stubs(:version).returns(MacOS::Version.new(10.4))
+    assert_equal LD64Dependency.new, @d.build(:ld64)
+  end
+
+  def test_ld64_dep_leopard_or_newer
+    MacOS.stubs(:version).returns(MacOS::Version.new(10.5))
+    assert_nil @d.build(:ld64)
+  end
+
   def test_raises_typeerror_for_unknown_classes
     assert_raises(TypeError) { @d.add(Class.new) }
   end