Gaffer17build fixes

[vfxenjoyer]

• Initialized usdLibs in the SConstruct before usage. This was causing the build to fail when GAFFERUSD was set to false, which we do sometimes at IE.
• Checked for existence of an env var before deleting it in __gaffer.py. This was causing builds at IE to fail because we don't set any value for LD_PRELOAD in our builds.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.