Add Kokkos::all_libs alias target for compatibility with TriBITS/Tr…

…ilinos (kokkos#6157)

* Add Kokkos::all_libs alias target for compatibility with Trilinos

* Add comments and fixup for cmake version less than 3.18

CMakeLists.txt

@@ -323,6 +323,8 @@ IF (NOT KOKKOS_HAS_TRILINOS AND NOT Kokkos_INSTALL_TESTING)
   #Make sure in-tree projects can reference this as Kokkos::
   #to match the installed target names
   ADD_LIBRARY(Kokkos::kokkos ALIAS kokkos)
+  # all_libs target is required for TriBITS-compliance
+  ADD_LIBRARY(Kokkos::all_libs ALIAS kokkos)
   TARGET_LINK_LIBRARIES(kokkos INTERFACE ${KOKKOS_COMPONENT_LIBRARIES})
   KOKKOS_INTERNAL_ADD_LIBRARY_INSTALL(kokkos)
 ENDIF()

cmake/KokkosConfigCommon.cmake.in

@@ -6,6 +6,17 @@ SET(Kokkos_CXX_COMPILER "@CMAKE_CXX_COMPILER@")
 SET(Kokkos_CXX_COMPILER_ID "@KOKKOS_CXX_COMPILER_ID@")
 SET(Kokkos_CXX_STANDARD @KOKKOS_CXX_STANDARD@)
 
+# Required to be a TriBITS-compliant external package
+IF(NOT TARGET Kokkos::all_libs)
+  # CMake Error at <prefix>/lib/cmake/Kokkos/KokkosConfigCommon.cmake:10 (ADD_LIBRARY):
+  #   ADD_LIBRARY cannot create ALIAS target "Kokkos::all_libs" because target
+  #   "Kokkos::kokkos" is imported but not globally visible.
+  IF(CMAKE_VERSION VERSION_LESS "3.18")
+    SET_TARGET_PROPERTIES(Kokkos::kokkos PROPERTIES IMPORTED_GLOBAL ON)
+  ENDIF()
+  ADD_LIBRARY(Kokkos::all_libs ALIAS Kokkos::kokkos)
+ENDIF()
+
 # Export Kokkos_ENABLE_<BACKEND> for each backend that was enabled.
 # NOTE: "Devices" is a little bit of a misnomer here.  These are really
 # backends, e.g. Kokkos_ENABLE_OPENMP, Kokkos_ENABLE_CUDA, Kokkos_ENABLE_HIP,