Prevent deps of build-time deps from leaking into the build environment

When decided what dependencies should be part of the build environment
(and have appropriate entries added to variables like PKG_CONFIG_PATH),
we select the entire dependency tree except for

 (1) inactive optional and recommended deps (2) indirect build-time deps
 (i.e., build-time deps of other deps)

There is a third category that sould be excluded: dependencies of direct
build-time deps. These are irrelevant to the build, and including them
can cause unexpected linkages.

Library/Homebrew/build.rb

@@ -100,6 +100,8 @@ class Build
         Dependency.prune
       elsif dep.build? && dependent != f
         Dependency.prune
+      elsif dep.build?
+        Dependency.keep_but_prune_recursive_deps
       end
     end
   end

Library/Homebrew/dependency.rb

@@ -82,6 +82,8 @@ class Dependency
           next []
         when :skip
           expand(dep.to_formula, &block)
+        when :keep_but_prune_recursive_deps
+          [dep]
         else
           expand(dep.to_formula, &block) << dep
         end
@@ -110,6 +112,11 @@ class Dependency
       throw(:action, :skip)
     end
 
+    # Keep a dependency, but prune its dependencies
+    def keep_but_prune_recursive_deps
+      throw(:action, :keep_but_prune_recursive_deps)
+    end
+
     def merge_repeats(deps)
       grouped = deps.group_by(&:name)
 

Library/Homebrew/test/test_dependency_expansion.rb

@@ -95,4 +95,17 @@ class DependencyExpansionTests < Test::Unit::TestCase
 
     assert_equal [@bar, @baz], deps
   end
+
+  def test_keep_dep_but_prune_recursive_deps
+    f = stub(:deps => [
+      build_dep(:foo, [:build], [@bar]),
+      build_dep(:baz, [:build]),
+    ])
+
+    deps = Dependency.expand(f) do |dependent, dep|
+      Dependency.keep_but_prune_recursive_deps if dep.build?
+    end
+
+    assert_equal [@foo, @baz], deps
+  end
 end