Add support for optional and recommended deps

Optional deps are not installed by default but generate a corresponding
"with-foo" option for the formula. Recommended deps _are_ installed by
default, and generate a corresponding "without-foo" option.

Library/Homebrew/build_options.rb

@@ -40,6 +40,20 @@ class BuildOptions
     @args.include? '--' + name
   end
 
+  def with? name
+    if has_option? "with-#{name}"
+      include? "with-#{name}"
+    elsif has_option? "without-#{name}"
+      not include? "without-#{name}"
+    else
+      false
+    end
+  end
+
+  def without? name
+    not with? name
+  end
+
   def head?
     @args.flag? '--HEAD'
   end

Library/Homebrew/formula.rb

@@ -829,6 +829,15 @@ private
     # This method is called once by `factory` before creating any instances.
     # It allows the DSL to finalize itself, reducing complexity in the constructor.
     def finalize_dsl
+      # Synthesize options for optional dependencies
+      dependencies.deps.select(&:optional?).each do |dep|
+        option "with-#{dep.name}", "Build with #{dep.name} support"
+      end
+
+      # Synthesize options for recommended dependencies
+      dependencies.deps.select(&:recommended?).each do |dep|
+        option "without-#{dep.name}", "Build without #{dep.name} support"
+      end
     end
   end
 end

Library/Homebrew/test/test_build_options.rb

@@ -27,6 +27,13 @@ class BuildOptionsTests < Test::Unit::TestCase
     assert !@build.include?("--with-foo")
   end
 
+  def test_with_without
+    assert @build.with?("foo")
+    assert @build.with?("bar")
+    assert @build.with?("baz")
+    assert @build.without?("qux")
+  end
+
   def test_used_options
     assert @build.used_options.include?("--with-foo")
     assert @build.used_options.include?("--with-bar")