Remove special cases in build-time dep audit

Closes Homebrew/homebrew#23931.

Library/Homebrew/cmd/audit.rb

@@ -151,11 +151,7 @@ class FormulaAuditor
 
       case dep.name
       when *BUILD_TIME_DEPS
-        next if dep.build?
-        next if dep.name == 'autoconf' && f.name =~ /automake/
-        next if dep.name == 'libtool' && %w{imagemagick libgphoto2 libp11 libextractor}.any? { |n| f.name == n }
-        next if dep.name =~ /autoconf|pkg-config/ && f.name == 'ruby-build'
-
+        next if dep.build? or dep.run?
         problem %{#{dep} dependency should be "depends_on '#{dep}' => :build"}
       when "git", "ruby", "emacs", "mercurial"
         problem <<-EOS.undent

Library/Homebrew/dependency_collector.rb

@@ -132,15 +132,16 @@ class DependencyCollector
   end
 
   def autotools_dep(spec, tags)
-    unless MacOS::Xcode.provides_autotools?
-      case spec
-      when :libltdl then spec = :libtool
-      else tags << :build
+    return if MacOS::Xcode.provides_autotools?
+
+    if spec == :libltdl
+      spec = :libtool
+      tags << :run
     end
 
+    tags << :build unless tags.include? :run
     Dependency.new(spec.to_s, tags)
   end
-  end
 
   def ant_dep(spec, tags)
     if MacOS.version >= :mavericks