[COOK-4468] Only set ENV variable when needed

When setting the environment variable to use system libraries,
'nokogiri' will use system libraries no matter what value the varible is
set to.

Also, the latest versions of `nokogiri` fail to compile with system
libraries on ubuntu[1].

[1] https://github.com/sparklemotion/nokogiri/issues/1099
Updated nokogiri workaround.

Default to the latest nokogiri by default, but if syslibs is defined to
be used, we want to check the version and make sure it's a working
version with syslibs.
Do checks on Ubuntu

This appears to be an Ubuntu issue, so this fix will do specific version
checking and workaround on Ubuntu only.
Simplified logic, and force syslib logic
Merge pull request #14 from gondoi/nokogiri

[COOK-4468] Only set ENV variable when needed