which are present starting in Nokogiri v1.11.0.rc4.

See sparklemotion/nokogiri#2145 for more information.

With this change, here's what compilation looks like when Nokogiri is
built with libxml2:

> /home/flavorjones/.rvm/rubies/ruby-2.7.2/bin/ruby -I. ../../../../ext/nokogumbo/extconf.rb
> checking for whether -I/home/flavorjones/.rvm/gems/ruby-2.7.2/gems/nokogiri-1.11.0.rc3/ext/nokogiri is accepted as CFLAGS... yes
> checking for whether -I/home/flavorjones/.rvm/gems/ruby-2.7.2/gems/nokogiri-1.11.0.rc3/ext/nokogiri/include is accepted as CFLAGS... yes
> checking for whether -I/home/flavorjones/.rvm/gems/ruby-2.7.2/gems/nokogiri-1.11.0.rc3/ext/nokogiri/include/libxml2 is accepted as CFLAGS... yes
> checking for libxml/tree.h... yes
> checking for nokogiri.h... yes
> creating Makefile

and here's what compilation looks like when Nokogiri is _not_ built with
libxml2:

> checking for whether -I/home/flavorjones/.rvm/gems/ruby-2.7.2/gems/nokogiri-1.11.0.rc3/ext/nokogiri is accepted as CFLAGS... yes
> checking for libxml/tree.h... no
> checking for nokogiri.h... no
> checking for xmlNewDoc() in -lxml2... yes
> checking for nokogiri.h in /home/flavorjones/.rvm/gems/ruby-2.7.2/gems/nokogiri-1.11.0.rc3/ext/nokogiri... yes
> creating Makefile

In a future update, once we've pinned the Nokogiri dependency to "~>
1.11", we should be able to remove the stanza that looks at
`libxml2_path`.

This is necessary on Windows where unresolved symbols aren't
allowed. We also limit compatibility with Nokogiri's precompiled libraries
to Nokogiri >= 1.11.2 on Windows for this reason.

Related to:
- sparklemotion/nokogiri#2145
- sparklemotion/nokogiri#2167
- sparklemotion/nokogiri#2202