build(ExternalLibs/mdspan): prefer find_package over fetchcontent (#653)

Closes #652

Author: pratikpc

CMakeLists.txt

@@ -125,7 +125,10 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
 endif()
 include(GNUInstallDirs)
 include(CMakePackageConfigHelpers)
-set(INSTALL_TARGETS ${TREELITE_TARGETS} objtreelite rapidjson mdspan)
+set(INSTALL_TARGETS ${TREELITE_TARGETS} objtreelite rapidjson)
+if(NOT mdspan_FOUND) # Found mdspan via FetchContent
+  list(APPEND INSTALL_TARGETS mdspan)
+endif()
 if(NOT nlohmann_json_FOUND)  # Fetched nlohmann/json via FetchContent
   list(APPEND INSTALL_TARGETS nlohmann_json)
 endif()

cmake/ExternalLibs.cmake

@@ -41,21 +41,24 @@ if(NOT nlohmann_json_FOUND)
 endif()
 
 # mdspan (header-only library)
-message(STATUS "Fetching mdspan...")
-set(MDSPAN_CXX_STANDARD 17 CACHE STRING "")
-FetchContent_Declare(
-  mdspan
-  GIT_REPOSITORY https://github.com/kokkos/mdspan.git
-  GIT_TAG        mdspan-0.6.0
-)
-FetchContent_GetProperties(mdspan)
-if(NOT mdspan_POPULATED)
-  FetchContent_Populate(mdspan)
-  add_subdirectory(${mdspan_SOURCE_DIR} ${mdspan_BINARY_DIR} EXCLUDE_FROM_ALL)
-  message(STATUS "mdspan was downloaded at ${mdspan_SOURCE_DIR}.")
-endif()
-if(MSVC)  # workaround for MSVC 19.x: https://github.com/kokkos/mdspan/issues/276
-  target_compile_options(mdspan INTERFACE "/permissive-")
+find_package(mdspan 0.6.0)
+if(NOT mdspan_FOUND)
+  message(STATUS "Fetching mdspan...")
+  set(MDSPAN_CXX_STANDARD 17 CACHE STRING "")
+  FetchContent_Declare(
+    mdspan
+    GIT_REPOSITORY https://github.com/kokkos/mdspan.git
+    GIT_TAG        mdspan-0.6.0
+  )
+  FetchContent_GetProperties(mdspan)
+  if(NOT mdspan_POPULATED)
+    FetchContent_Populate(mdspan)
+    add_subdirectory(${mdspan_SOURCE_DIR} ${mdspan_BINARY_DIR} EXCLUDE_FROM_ALL)
+    message(STATUS "mdspan was downloaded at ${mdspan_SOURCE_DIR}.")
+  endif()
+  if(MSVC)  # workaround for MSVC 19.x: https://github.com/kokkos/mdspan/issues/276
+    target_compile_options(mdspan INTERFACE "/permissive-")
+  endif()
 endif()
 
 # Google C++ tests