cmake/install: add THRUST_ENABLE_INSTALL_RULES option

cmake/install: mv ThrustAddSubdir handling to after project()

We want to add the option to install in this case, which requires
the paths set by project()

thrust: separate out determination of whether Thrust is top-level project

This determines whether an install target will be provided. If Thrust is
built as top-level project, the install target will be provided by default,
as before.

If Thrust is built as a sub-project, the install target will, by default,
not be provided, again maintaining existing behavior.

So what's new here is that via this option a downstream project can enable
install of thrust if it is used via `add_subdirectory`.

Update CONTRIBUTING.md with new CMake option docs.

CMakeLists.txt

@@ -1,10 +1,3 @@
-# Support adding Thrust to a parent project via add_subdirectory.
-# See examples/cmake/add_subdir/CMakeLists.txt for details.
-if (NOT "${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_LIST_DIR}")
-  include(cmake/ThrustAddSubdir.cmake)
-  return()
-endif()
-
 # 3.15 is the minimum.
 # 3.17 for nvc++/Feta
 # 3.18 for C++17 + CUDA
@@ -18,12 +11,31 @@ endif()
 
 project(Thrust NONE)
 
+# Determine whether Thrust is the top-level project or included into
+# another project via add_subdirectory()
+if ("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_LIST_DIR}")
+  set(THRUST_TOPLEVEL_PROJECT ON)
+else()
+  set(THRUST_TOPLEVEL_PROJECT OFF)
+endif()
+
+option(THRUST_ENABLE_INSTALL_RULES "Enable installation of Thrust" ${THRUST_TOPLEVEL_PROJECT})
+if (THRUST_ENABLE_INSTALL_RULES)
+  include(cmake/ThrustInstallRules.cmake)
+endif()
+
+# Support adding Thrust to a parent project via add_subdirectory.
+# See examples/cmake/add_subdir/CMakeLists.txt for details.
+if (NOT THRUST_TOPLEVEL_PROJECT)
+  include(cmake/ThrustAddSubdir.cmake)
+  return()
+endif()
+
 include(cmake/AppendOptionIfAvailable.cmake)
 
 include(cmake/ThrustBuildCompilerTargets.cmake)
 include(cmake/ThrustBuildTargetList.cmake)
 include(cmake/ThrustMultiConfig.cmake)
-include(cmake/ThrustInstallRules.cmake)
 include(cmake/ThrustUtilities.cmake)
 
 option(THRUST_ENABLE_HEADER_TESTING "Test that all public headers compile." "ON")

CONTRIBUTING.md

@@ -336,6 +336,8 @@ The CMake options are divided into these categories:
 - `THRUST_ENABLE_EXAMPLE_FILECHECK={ON, OFF}`
   - Enable validation of example outputs using the LLVM FileCheck utility.
     Default is `OFF`.
+- `THRUST_ENABLE_INSTALL_RULES={ON, OFF}`
+  - If true, installation rules will be generated for thrust. Default is `ON`.
 
 ## Single Config CMake Options
 
@@ -391,6 +393,10 @@ The CMake options are divided into these categories:
     simultaneously.
   - CUB configurations will be generated for each C++ dialect targeted by
     the current Thrust build.
+- `THRUST_INSTALL_CUB_HEADERS={ON, OFF}`
+  - If enabled, the CUB project's headers will be installed through Thrust's
+    installation rules. Default is `ON`.
+  - This option depends on `THRUST_ENABLE_INSTALL_RULES`.
 - `THRUST_ENABLE_COMPUTE_XX={ON, OFF}`
   - Controls the targeted CUDA architecture(s)
   - Multiple options may be selected when using NVCC as the CUDA compiler.