Add support for CTest stdout compression in XML reports

recent versions of CMake/CTest use compression in the XML format parsed by CTestXML2HTML (i.e. make target HTMLSummary).

This change add support for it, so that the HTML reports are usable.

I also made a couple of improvements to GitLab-CI configuration:

• set CCACHE_CCP2 so that ccache passes the original .cpp file to the compiler (needed to avoid warnings relying on comments or indentation). Note that the default changed with ccache 3.3, but LCG still uses 3.2.5
• keep a copy of CTest XML summary in test artifacts

See merge request !453 (merged)

• [2017-10-25 14:22] Automatic merge failed in lhcb-gaudi-merge#259

merged

mentioned in commit 86eed8ba