Modernisation of CMake scripting to simplify build of FullSimLight

With Athena now using Geant4 static/big libraries, optimization work has shifted to look at use of LTO/PGO. As these are relatively involved tasks, a first step is to use FullSimLight as a proof-of-principle for the flags and operations needed to support this. Though this work won't (at least at this stage) be integrated into GeoModel/FullSimLight, the need to hack at the build scripts has highlighted some modernizations/simplifications that can be made independently right now. These are not only useful to assist the LTO/PGO work, but for general efficiency and clarity of the build.

• Bump CMake minimum to 3.16, tested up to 3.26. Could be reduced a little, but 3.16 or higher is now pretty common across all platforms (see Repology listing).
  • Allows removal of some workarounds needed with older versions
  • If FullSimLight also gets used to test use of Celeritas/AdePT GPU simulation libraries for ATLAS, a more modern CMake will be needed for better CUDA support.
• Consistently create namespaces ALIAS targets in subprojects. Only link to namespaced GeoModel targets in subprojects
  • Avoids need to check whether a project is built in a full GeoModel build or standalone.
• Make ExternalProject targets for JSON, Eigen, and Xerces-C dependencies of the internal imported targets
  • Avoids need to use add_dependencies on GeoModel targets that link to these. They simply link to the namespaced imported target, e.g. nlohmann_json::nlohmann_json and don't need to worry if this is coming from the builtin on the system.
• Fully implement the FullSimLight target as an INTERFACE target so plugins just need to link to it.
• Build the core of the FullSimLight application as an OBJECT library.
  • Applications just link to this instead of recompiling the sources each time for their own use.
• General simplifications:
  • Keep all build commands for a target as close together as possible for clarity
  • Check subproject status using CMAKE_CURRENT_SOURCE_DIR instead of an artificial project command.
  • Remove dead code or no longer required/supported modules (e.g. Geant4_USE_FILE)

These have been tested locally on macOS with as many combinations of build options as I can enable and appears fine. CI is more than likely to highlight issues, but let's see what it throws up.

FSL/CMakeLists.txt

-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.16...3.26)
 
 project(fsl VERSION 0.1 LANGUAGES CXX)
 
@@ -20,11 +20,7 @@ if(CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
          "Framework finding behaviour on macOS" )
     # Set up how the project handle some of its dependenices. Either by picking them
     # up from the environment, or building them itself.
-    if( NOT TARGET JSONExt )
-        include( SetupJSON )
-    endif()
-    # Set a flag to steer the  of the subpackages
-    set( ${PROJECT_NAME}_INDIVIDUAL_BUILD ON )
+    include( SetupJSON )
     find_package (FullSimLight REQUIRED)
     add_definitions (-DFSLPLUGINROOT="${FullSimLight_DIR}/../../../${CMAKE_INSTALL_DATAROOTDIR}" )
     message  ( STATUS "FSL plugin files sought here: ${FullSimLight_DIR}/../../../${CMAKE_INSTALL_DATAROOTDIR}" )
@@ -80,18 +76,7 @@ set(PROJECT_SOURCES ${sources} ${headers}  ${uis} )
 
 
 add_executable(fsl ${PROJECT_SOURCES} )
-
-
-target_link_libraries(fsl PRIVATE Qt5::Widgets)
-
-if( GEOMODEL_USE_BUILTIN_JSON )
-   add_dependencies(fsl JSONExt )
-endif()
- 
-if( nlohmann_json_FOUND )
-    target_link_libraries( fsl PRIVATE nlohmann_json::nlohmann_json )
-endif()
-  
+target_link_libraries(fsl PRIVATE Qt5::Widgets nlohmann_json::nlohmann_json)
 
 # Install the executable.
 install( TARGETS fsl