[ROOT-2747] Fixed comments in TGeoVolume.cxx for the reference guide

(cherry picked from commit 21a16dc056d9868986138e2329a134baa4d647cb)
Update another lookup ctx after AutoParse.
Try to load from PCM if available (ROOT-6487).
Building LLVM and CLING as CMake sub-directories instead of as ExternalProjects. Ninja is now supported.
Make the TAxis and TGaxis API consistent (SetTickSize() ands SetTickLength()). Forum request.
Disable MSVC warning in VC tests
Clean up all the "heavy comments" as requested here http://root.cern.ch/phpBB3/viewtopic.php?f=10&t=18352
Improve error messages.
Protection for correctly computing the bounding box when adding assemblies without content to another assembly.

(cherry picked from commit eeb676568becee8d5eb1de3e2a7c62bfa301bab8)
Fixed broken comment in TGeoVolume.cxx preventing displaying correctly the reference guide

(cherry picked from commit 329eedd11a2c688f01857dbcf39f6b6094d21f7c)
Neutralize ClassImp - the interpreter really does not care.
Modified the builtin externals dependencies to their libraries to make it work for Ninja.
Ensure that the test executables for not pollute the /bin and /lib directories also for Windows :-)
Ensure that $ROOTSYS/bin is in the PATH when running tests for Windows
Fix for ROOT-6464
Fix for ROOT-6396
Fix for ROOT-6363 (hard links not preserved on make install)
Make TNetXNGFile work with XRootD 4
Support for XRootD 4

This patch - together with  cc884ecb832f025af4dd5eea0eb27ad017e76b0d - adds support fro XRootD version 4.
The bulk of the patch copes with the changes in the XRootD network classes to support IPv6. The rest is mostly
handling of the changes in packaging.
The PROOF part still uses the old XrdClient (still delivered with XRootD).

Conflicts:
	configure
	proof/proofd/src/XrdProofdNetMgr.cxx
always clean up xproofd.tmp.