Mark symbol-style autotools deps as build deps by default

Library/Homebrew/dependency_collector.rb

@@ -76,7 +76,7 @@ private
     case spec
     when :autoconf, :automake, :bsdmake, :libtool
       # Xcode no longer provides autotools or some other build tools
-      Dependency.new(spec.to_s, tag) unless MacOS::Xcode.provides_autotools?
+      Dependency.new(spec.to_s, [:build, *tag]) unless MacOS::Xcode.provides_autotools?
     when *X11Dependency::Proxy::PACKAGES
       if MacOS.version >= :mountain_lion
         Dependency.new(spec.to_s, tag)