AnalysisBase Threads Fix, master branch (2020.05.25.)

In order to compile client code (or partially compile packages of this repository) against an AnalysisBase release/nightly, all imported targets used by the targets exported by AnalysisBase must be available.

This addition takes care of that, providing the Threads::Threads imported target to the CMake configuration of AnalysisBase.

Note that in the 21.2 branch this is instead done in the post-configuration file. (https://gitlab.cern.ch/atlas/athena/-/blob/21.2/Projects/AnalysisBase/PostConfig.cmake.in) But experience with imported targets and partial builds in master taught me that it's more robust to do it during "pre-configuration" instead.

Thanks to @nkoehler and @mkbugge for reporting the issue.

Projects/AnalysisBase/cmake/PreConfig.cmake.in

+# Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
+#
+# File setting up imported targets needed by the exported targets of
+# @CMAKE_PROJECT_NAME@.
+#
+
+# Figure out whether to use QUIET in the find_package call(s).
+set( _quietFlag )
+if( @CMAKE_PROJECT_NAME@_FIND_QUIETLY )
+  set( _quietFlag QUIET )
+endif()
+
+# Imported target(s) used by the project.
+find_package( Threads ${_quietFlag} )
+
+# Clean up.
+unset( _quietFlag )