fix build failure on darwin:
```
checking for xmlParseDoc() in -lxml2... no
checking for xmlParseDoc() in -llibxml2... no
checking for libxml-2.0 using `pkg_config`... no
Please install either the `pkg-config` utility or the `pkg-config` rubygem.
checking for xmlParseDoc() in -lxml2... no
checking for xmlParseDoc() in -llibxml2... no
-----
extconf.rb:295:in `ensure_package_configuration'
extconf.rb:704:in `<main>'
xml2 is missing. Please locate mkmf.log to investigate how it is failing.
```
annalee
c3d54620fd