Whoops, I lied, I think we do need them. I've reverted that change for now. I get the following error with linking when they are excluded.
```
/usr/local/google/home/jud/mod_pagespeed/src/net/instaweb/automatic/pagespeed_automatic.a(59.system_rewrite_driver_factory.o.o): In function `net_instaweb::SystemRewriteDriverFactory::InitStats(net_instaweb::Statistics*)':
system_rewrite_driver_factory.cc:(.text._ZN12net_instaweb26SystemRewriteDriverFactory9InitStatsEPNS_10StatisticsE+0x12): undefined reference to `net_instaweb::SerfUrlAsyncFetcher::InitStats(net_instaweb::Statistics*)'
objs/addon/src/ngx_rewrite_driver_factory.o: In function `net_instaweb::NgxRewriteDriverFactory::DefaultAsyncUrlFetcher()':
/usr/local/google/home/jud/ngx_pagespeed/src/ngx_rewrite_driver_factory.cc:154: undefined reference to `net_instaweb::SerfUrlAsyncFetcher::SerfUrlAsyncFetcher(char const*, apr_pool_t*, net_instaweb::ThreadSystem*, net_instaweb::Statistics*, net_instaweb::Timer*, long, net_instaweb::MessageHandler*)'
objs/addon/src/ngx_server_context.o: In function `net_instaweb::NgxServerContext::ApplySessionFetchers(net_instaweb::RefCountedPtr<net_instaweb::RequestContext> const&, net_instaweb::RewriteDriver*)':
/usr/local/google/home/jud/ngx_pagespeed/src/ngx_server_context.cc:133: undefined reference to `net_instaweb::LoopbackRouteFetcher::LoopbackRouteFetcher(net_instaweb::RewriteOptions const*, std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int, net_instaweb::UrlAsyncFetcher*)'
/usr/local/google/home/jud/ngx_pagespeed/src/ngx_server_context.cc:137: undefined reference to `net_instaweb::AddHeadersFetcher::AddHeadersFetcher(net_instaweb::RewriteOptions const*, net_instaweb::UrlAsyncFetcher*)'
collect2: ld returned 1 exit status
```