libxml2 should be compiled with "-O2"

[flavorjones]

The vendored libxml2 is being compiled with no optimization. This causes slow performance in some cases, e.g. #2020

This is caused by CFLAGS being set by default to -fPIC by process_recipe in ext/nokogiri/extconf.rb. When libxml2 is configured with any CFLAGS set, then it is compiled without optimization.

This issue is to ensure that libxml2 is compile with -O2 which appears to be the default in libxml2's configure script for gcc.

Final challenge: MiniPortile (and so process_recipe) requires that environment variables be passed as configure_options which means that later applications of, e.g. CFLAGS=something may override earlier applications. I think we should clean this up along the way, because it's making some things (like this) harder.

[flavorjones]

libxml2 should probably also be compiled with -g, which is also the default and being overridden by the vendored library build process.