Allow installing components when downstream

In the @acts-project we adopt an (admittedly somewhat unconventional)
build system in which software A depends on B, and B depends on CCCL.
The setup is that we want to install B into a prefix, and then try to
build A against B. The problem arises is that we are using CMake to
dynamically fetch CCCL using the so-called "FetchContent" mechanism,
which downloads CCCL and then adds it as a subdirectory.

The core problem is that installing software B which has included CCCL
does not actually install CCCL in the same prefix, so software A cannot
then load software B as CCCL is not installed. The reason this happens
is that CMakeLists.txt:28 (at the time of writing) returns from the
CMake configuration stage early, and leaves the CUB, Thrust, and
libcudacxx directories unincluded (see lines 70 to 72).

Although this is, again, an unconventional and rare scenario, it should
be easy to add support for this kind of build, and I hope the CCCL devs
would agree that it might be worth doing. In this commit, I remove the
early return and replace it with additional if-statements. This commit
should leave any existing workflows completely untouched, but should
make it easier to use CCCL in the way we do in @acts-project.

CMakeLists.txt

@@ -25,17 +25,18 @@ endif()
 # Support adding CCCL to a parent project via add_subdirectory.
 if (NOT CCCL_TOPLEVEL_PROJECT)
   include(cmake/CCCLAddSubdir.cmake)
-  return()
 endif()
 
 # We require a higher cmake version for dev builds
-cmake_minimum_required(VERSION 3.21)
+if (CCCL_TOPLEVEL_PROJECT)
+  cmake_minimum_required(VERSION 3.21)
+endif()
 
-option(CCCL_ENABLE_LIBCUDACXX "Enable the libcu++ developer build." ON)
-option(CCCL_ENABLE_CUB "Enable the CUB developer build." ON)
-option(CCCL_ENABLE_THRUST "Enable the Thrust developer build." ON)
-option(CCCL_ENABLE_TESTING "Enable CUDA C++ Core Library tests." ON)
-option(CCCL_ENABLE_EXAMPLES "Enable CUDA C++ Core Library examples." ON)
+option(CCCL_ENABLE_LIBCUDACXX "Enable the libcu++ developer build." ${CCCL_TOPLEVEL_PROJECT})
+option(CCCL_ENABLE_CUB "Enable the CUB developer build." ${CCCL_TOPLEVEL_PROJECT})
+option(CCCL_ENABLE_THRUST "Enable the Thrust developer build." ${CCCL_TOPLEVEL_PROJECT})
+option(CCCL_ENABLE_TESTING "Enable CUDA C++ Core Library tests." ${CCCL_TOPLEVEL_PROJECT})
+option(CCCL_ENABLE_EXAMPLES "Enable CUDA C++ Core Library examples." ${CCCL_TOPLEVEL_PROJECT})
 option(CCCL_ENABLE_BENCHMARKS "Enable CUDA C++ Core Library benchmarks." OFF)
 
 option(CCCL_ENABLE_UNSTABLE "Enable targets and developer build options for unstable projects." OFF)
@@ -44,27 +45,30 @@ if (CCCL_ENABLE_UNSTABLE)
   option(CCCL_ENABLE_CUDAX "Enable the CUDA Experimental developer build." ON)
 endif()
 
+if (CCCL_ENABLE_TESTING)
+  include(CTest)
+  enable_testing()
+endif()
 
-include(CTest)
-enable_testing()
-
-include(cmake/CCCLUtilities.cmake) # include this first
-include(cmake/CCCLClangdCompileInfo.cmake)
+if (CCCL_TOPLEVEL_PROJECT)
+  include(cmake/CCCLUtilities.cmake) # include this first
+  include(cmake/CCCLClangdCompileInfo.cmake)
+endif()
 
 if (CCCL_ENABLE_LIBCUDACXX)
-  set(LIBCUDACXX_TOPLEVEL_PROJECT ON)
+  set(LIBCUDACXX_TOPLEVEL_PROJECT ${CCCL_TOPLEVEL_PROJECT})
 endif()
 
 if (CCCL_ENABLE_CUB)
-  set(CUB_TOPLEVEL_PROJECT ON)
+  set(CUB_TOPLEVEL_PROJECT ${CCCL_TOPLEVEL_PROJECT})
 endif()
 
 if (CCCL_ENABLE_THRUST)
-  set(THRUST_TOPLEVEL_PROJECT ON)
+  set(THRUST_TOPLEVEL_PROJECT ${CCCL_TOPLEVEL_PROJECT})
 endif()
 
 if (CCCL_ENABLE_CUDAX)
-  set(cudax_TOPLEVEL_PROJECT ON)
+  set(cudax_TOPLEVEL_PROJECT ${CCCL_TOPLEVEL_PROJECT})
 endif()
 
 add_subdirectory(libcudacxx)