Add :libltdl dep for libtool's libltdl

The :libtool dep is implicitly a build-time dep; :libltdl can be used
for software that uses the ltdl library at runtime.

Library/Homebrew/dependency_collector.rb

@@ -74,7 +74,7 @@ private
 
   def parse_symbol_spec spec, tag
     case spec
-    when :autoconf, :automake, :bsdmake, :libtool
+    when :autoconf, :automake, :bsdmake, :libtool, :libltdl
       # Xcode no longer provides autotools or some other build tools
       autotools_dep(spec, tag)
     when *X11Dependency::Proxy::PACKAGES
@@ -103,8 +103,13 @@ private
   end
 
   def autotools_dep(spec, tag)
+    case spec
+    when :libltdl then spec, tag = :libtool, Array(tag)
+    else tag = Array(tag) << :build
+    end
+
     unless MacOS::Xcode.provides_autotools?
-      Dependency.new(spec.to_s, [:build, *tag])
+      Dependency.new(spec.to_s, tag)
     end
   end
 end

Library/Homebrew/test/test_dependency_collector.rb

@@ -82,6 +82,13 @@ class DependencyCollectorTests < Test::Unit::TestCase
     assert dep.optional?
   end
 
+  def test_libltdl_not_build_dep
+    MacOS::Xcode.stubs(:provides_autotools?).returns(false)
+    dep = @d.build(:libltdl)
+    assert_equal Dependency.new("libtool"), dep
+    assert !dep.build?
+  end
+
   def test_autotools_dep_no_system_autotools
     MacOS::Xcode.stubs(:provides_autotools?).returns(false)
     dep = @d.build(:libtool)