libxml2: don't set LIBS environment variable for autotools

[madebr]

This avoid linking errors when doing test compiles when doing '-o *:shared=True'

Specify library name and version: libxml2/all

• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the
  conan-center hook activated.

Fixes #3000

[chhitz]

Can somebody check why the tests are failing? We're blocked on this issue.

[madebr]

The logs are created: https://c3i.jfrog.io/artifactory/misc/logs/pr/3080/

[ghost]

I detected other pull requests that are modifying libxml2 recipe:

• libxml2: fix mingw on Linux + double quotes + libxml2 does not create cmake targets #2714

This message is automatically generated by https://github.com/ericLemanissier/conan-center-conflicting-prs so don't hesitate to report issues/improvements there.

[conan-center-bot]

All green in build 4 (484893459e85b5223acd247f58253c96ac3535d9)! 😊

• libxml2/2.9.9: Generated 88 packages. All logs here
• libxml2/2.9.10: Generated 88 packages. All logs here