build: Unconditionally load Boost in CMake config (#3779)

Currently, as @krasznaa discovered, the finding of the Boost dependency in an installed version of Acts (i.e., through `ActsConfig.cmake` uses the `@ACTS_USE_SYSTEM_BOOST@` template, but this variable no longer exists.

CMake, in its infinite wisdom, silently ignores this, producing `if()`, which it _again_ silently accepts and which apparently evaluates to false, so Boost is never loaded.

This commit fixes the issue by ensuring that Boost is loaded unconditionally.

cmake/ActsConfig.cmake.in

@@ -52,9 +52,7 @@ list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/Modules)
 # `find_dependency` is a wrapper around `find_package` that automatically
 # handles QUIET and REQUIRED parameters.
 include(CMakeFindDependencyMacro)
-if(@ACTS_USE_SYSTEM_BOOST@)
-  find_dependency(Boost @Boost_VERSION_STRING@ CONFIG EXACT)
-endif()
+find_dependency(Boost @Boost_VERSION_STRING@ CONFIG EXACT)
 if(@ACTS_USE_SYSTEM_EIGEN3@)
   find_dependency(Eigen3 @Eigen3_VERSION@ CONFIG EXACT)
 endif()
@@ -86,10 +84,6 @@ endif()
 
 # dependencies that we have built ourselves but cannot be
 # straightforwardly handed to cmake
-if(NOT @ACTS_USE_SYSTEM_BOOST@)
-  add_library(Boost::boost INTERFACE IMPORTED GLOBAL)
-  target_include_directories(Boost::boost INTERFACE "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}")
-endif()
 if(NOT @ACTS_USE_SYSTEM_EIGEN3@)
   add_library(Eigen3::Eigen INTERFACE IMPORTED GLOBAL)
   target_include_directories(Eigen3::Eigen INTERFACE "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}")

thirdparty/README.md

@@ -17,9 +17,7 @@ CMake instructions to build [nlohmann::json](https://github.com/nlohmann/json).
 ## boost
 
 For convenience, it's possible to use the ACTS build system to build the minimum
-required version of [boost](https://www.boost.org/) (currently 1.71.0).  No source is
-bundled here, and if requested via "-DACTS_USE_SYSTEM_BOOST=OFF", only the filesystem,
-program_options, and test libraries will be built.
+required version of [boost](https://www.boost.org/) (currently 1.71.0).
 
 Warning: during installation, the built boost libraries will be installed alongside the
 ACTS libraries, with a version suffix. This location may be known to the system linker.