cmake: Install the OCaml libraries in a package subdirectory

[mgorny]

Summary

Install the OCaml libraries in a dedicated subdirectory of lib/ocaml rather than top-level.

The top-level directory is reserved for the standard library, and has precedence over local directory in search path. As a result, OCaml preferred the files installed along with previous LLVM version over the source tree when building a new version, resulting in two versions being mixed during the build.

The subdirectory layout is the standard layout used by other OCaml packages, and findlib is able to find the LLVM libraries successfully in the subdirectory.

Bug: https://bugs.gentoo.org/559624

[mgorny]

Will be folded into https://reviews.llvm.org/D24354

cmake/modules/AddOCaml.cmake

if( NOT (ext STREQUAL ".cmo" OR
              ext STREQUAL CMAKE_SHARED_LIBRARY_SUFFIX) )
       list(APPEND install_files "${ocaml_output}")
     elseif( ext STREQUAL CMAKE_SHARED_LIBRARY_SUFFIX)
       list(APPEND install_shlibs "${ocaml_output}")
     endif()
   endforeach()
 
   install(FILES ${install_files}
-          DESTINATION lib/ocaml)
+          DESTINATION lib/ocaml/llvm)
   install(FILES ${install_shlibs}
           PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE
                       GROUP_READ GROUP_EXECUTE
                       WORLD_READ WORLD_EXECUTE
-          DESTINATION lib/ocaml)
+          DESTINATION lib/ocaml/llvm)
 
   foreach( install_file ${install_files} ${install_shlibs} )
     get_filename_component(filename "${install_file}" NAME)
     add_custom_command(TARGET "ocaml_${name}" POST_BUILD
       COMMAND "${CMAKE_COMMAND}" "-E" "copy" "${install_file}"
                                              "${LLVM_LIBRARY_DIR}/ocaml/"
       COMMENT "Copying OCaml library component ${filename} to intermediate area"
       VERBATIM)
   endforeach()
 endfunction()
 
 add_custom_target("ocaml_all")