This picks up the dmtcp package need by AthSimulation. It also makes
AtlasLCG a bit more robust. Fixes ATLASSIM-3342

The 1.0.14 tag of atlasexternals should pick up the fixes to `findGaudi.cmake`
discussed originally in ATLINFR-1699 and also in ATLINFR-1711.

It should avoid the issues introduced in 1.0.13 as discussed in ATLINFR-1769.

To:
  - Use LCG_88 from now;
  - Pick up the build of the dmtcp external for Athena;
  - Include the latest dictionary generation updates for the CMake code.

The 1.0.12 tag doesn't actually exist at the time of this commit just
yet, but by the time a merge request will be set up, it will...

level to -O2 in Release mode. Since as discussed in ATLINFR-1642, the default
-O3 setting causes some problems in the q-tests.

Fixing the issue with FindXrootd.cmake not adding the XRootD executables
to $PATH.

specific tag of the atlasexternals repository. Which should provide
the same externals that were built into the SVN based
AtlasExternals-21.0.20.

the atlasexternals repository. To get GCC checker warnings again, and to
store the exact version of the repository used for the build, in the
ReleaseData files generated for the projects.

atlasexternals repository instead of the commit hash that was used for
the previous testing round.

of the atlasexternals repository. To fix the ZSH issues that showed up in the
Athena build, and to bring the AthenaP1 build up to date.

Since the new tag introduces a different behaviour for the package filter file,
I had to tweak the AthenaP1 filter file a bit. (The Athena file was already
updated to work correctly with the new behaviour.)

build setup. Local tests showed the build working okay on a few key
packages with this setup.

set of externals. And to use a different release number.