Do not include distributed dir in ArborX target without MPI

[aprokop]

Currently building against ArborX installed without MPI is broken:

  Imported target "ArborX::ArborX" includes non-existent path

    "<install_dir>/include/ArborX/distributed"

This is because when we don't enable MPI, none of the files from src/distributed are getting installed, so distributed directory in include is not created. This is likely been broken since #1167.

Somehow, our testing did not catch it. There is one build that has MPI off, CUDA-11.0.3-Clang. I don't understand why that one does not fail. In the install, it clearly does not install any files in distributed include. Perhaps, it is not always emitted (like, here).

[dalg24]

I am tempted to say to do this instead

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 163291c5..abb6d8ec 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -145,14 +145,8 @@ install(FILES
   ${CMAKE_CURRENT_BINARY_DIR}/ArborXSettings.cmake
   DESTINATION ${CMAKE_INSTALL_DATADIR}/cmake/ArborX )

-if(ARBORX_ENABLE_MPI)
-  install(DIRECTORY ${PROJECT_SOURCE_DIR}/src/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/ArborX
-          FILES_MATCHING PATTERN "*.hpp")
-else()
-  install(DIRECTORY ${PROJECT_SOURCE_DIR}/src/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/ArborX
-          FILES_MATCHING PATTERN "*.hpp"
-          PATTERN "*Distribut*" EXCLUDE)
-endif()
+install(DIRECTORY ${PROJECT_SOURCE_DIR}/src/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/ArborX
+        FILES_MATCHING PATTERN "*.hpp")
 install(DIRECTORY ${PROJECT_BINARY_DIR}/include/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/ArborX
         FILES_MATCHING PATTERN "*.hpp")

[aprokop]

I am tempted to say to do this instead

OK, let's try this instead.