Relax required CMake version back to 3.17

CMakeLists.txt

@@ -20,7 +20,7 @@
 # STRICT LIABILITY, OR TOR (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-cmake_minimum_required(VERSION 3.21)
+cmake_minimum_required(VERSION 3.17)
 
 PROJECT(tiny-cuda-nn
 	VERSION 1.1
@@ -64,9 +64,11 @@ set(CMAKE_CUDA_STANDARD_REQUIRED ON)
 set(CMAKE_CUDA_EXTENSIONS OFF)
 set(CUDA_LINK_LIBRARIES_KEYWORD PUBLIC)
 
+get_directory_property(TCNN_HAS_PARENT PARENT_DIRECTORY)
+
 if (DEFINED ENV{TCNN_CUDA_ARCHITECTURES})
 	message(STATUS "Obtained target architecture from environment variable TCNN_CUDA_ARCHITECTURES=$ENV{TCNN_CUDA_ARCHITECTURES}")
-	if (NOT PROJECT_IS_TOP_LEVEL)
+	if (TCNN_HAS_PARENT)
 		set(CMAKE_CUDA_ARCHITECTURES $ENV{TCNN_CUDA_ARCHITECTURES} PARENT_SCOPE)
 	endif()
 	set(CMAKE_CUDA_ARCHITECTURES $ENV{TCNN_CUDA_ARCHITECTURES})
@@ -77,7 +79,7 @@ else()
 	string(STRIP "${INSTALLED_GPU_CCS_1}" INSTALLED_GPU_CCS_2)
 	string(REPLACE " " ";" INSTALLED_GPU_CCS_3 "${INSTALLED_GPU_CCS_2}")
 	string(REPLACE "." "" CUDA_ARCH_LIST "${INSTALLED_GPU_CCS_3}")
-	if (NOT PROJECT_IS_TOP_LEVEL)
+	if (TCNN_HAS_PARENT)
 		set(CMAKE_CUDA_ARCHITECTURES ${CUDA_ARCH_LIST} PARENT_SCOPE)
 	endif()
 	set(CMAKE_CUDA_ARCHITECTURES ${CUDA_ARCH_LIST})