Protection against extremely small 2D histograms' zoom box.
Suggested here: https://sft.its.cern.ch/jira/browse/ROOT-6673
Do not auto replace explicitly set, default sized TTreeCache (ROOT-6698).
Add a protection to avoid the crash mentionned here: https://sft.its.cern.ch/jira/browse/ROOT-6675
Changes provided by Patricia to fix the ability to locate some external packages with CMake