Taught AnalysisBase to set up Threads::Threads for client projects.

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.

Projects/AnalysisBase/CMakeLists.txt

@@ -34,10 +34,13 @@ atlas_ctest_setup()
 atlas_project( USE AnalysisBaseExternals ${AnalysisBaseExternals_VERSION}
    PROJECT_ROOT ${CMAKE_SOURCE_DIR}/../../ )
 
-# Configure and install the post-configuration file:
+# Configure and install the pre/post-configuration files:
+configure_file( ${CMAKE_SOURCE_DIR}/cmake/PreConfig.cmake.in
+   ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/PreConfig.cmake @ONLY )
 configure_file( ${CMAKE_SOURCE_DIR}/cmake/PostConfig.cmake.in
    ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/PostConfig.cmake @ONLY )
-install( FILES ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/PostConfig.cmake
+install( FILES ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/PreConfig.cmake
+               ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/PostConfig.cmake
    DESTINATION ${CMAKE_INSTALL_CMAKEDIR} )
 
 # Generate replacement rules for the installed paths:

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 )