lighttpd: run autogen.sh to ensure configure works

Fixes Homebrew/homebrew#25649.

Formula/lighttpd.rb

@@ -10,6 +10,7 @@ class Lighttpd < Formula
   depends_on 'pkg-config' => :build
   depends_on 'autoconf' => :build
   depends_on 'automake' => :build
+  depends_on 'libtool' => :build
   depends_on 'pcre'
   depends_on 'lua' => :optional
   depends_on 'libev' => :optional
@@ -39,6 +40,9 @@ class Lighttpd < Formula
     # fixed upstream, should be in next release: http://redmine.lighttpd.net/issues/2517
     inreplace 'src/Makefile.am', '$(LDAP_LIB)', '$(SSL_LIB) $(LDAP_LIB)'
 
+    # autogen must be run, otherwise prebuilt configure may complain
+    # about a version mismatch between included automake and Homebrew's
+    system "./autogen.sh"
     system "./configure", *args
     system "make install"