Do not veto xrootd tutorials when using built-in xrootd

[eguiraud]

XROOTD_FOUND is only set if using external xrootd.
xrootd is set by both external xrootd and built-in xrootd.

[phsft-bot]

Starting build on ROOT-debian10-i386/cxx14, ROOT-performance-centos7-multicore/default, ROOT-fedora29/python3, ROOT-fedora30/cxx14, ROOT-fedora31/noimt, ROOT-ubuntu16/nortcxxmod, mac1015/cxx17, windows10/cxx14
How to customize builds

[stwunsch]

👍

[phsft-bot]

Build failed on ROOT-performance-centos7-multicore/default.
Running on olhswep22.cern.ch:/data/sftnight/workspace/root-pullrequests-build
See cdash.
See console output.

Errors:

• [2020-05-06T11:36:29.817Z] 652/2022 Test [TDF] minor fixes #370: tutorial-dataframe-df103_NanoAODHiggsAnalysis .....................................................***Failed Error regular expression found in output. Regex=[: error:] 4.94 sec

Failing tests:

• projectroot.runtutorials.tutorial_dataframe_df103_NanoAODHiggsAnalysis

[eguiraud]

failures in projectroot.runtutorials.tutorial_dataframe_df103_NanoAODHiggsAnalysis are understood, they were hidden by the bug that this PR fixes, and should be fixed by #5554

[phsft-bot]

Build failed on ROOT-debian10-i386/cxx14.
Running on pcepsft10.dyndns.cern.ch:/build/workspace/root-pullrequests-build
See cdash.
See console output.

Errors:

• [2020-05-06T15:41:22.686Z] CMake Error at /home/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:914 (message):