cmake: Make OCaml library install path configurable

[mgorny]

Summary

Add a OCAML_INSTALL_PATH variable that can be used to override the install path for OCaml libraries. It allows overriding the path if the new defaults are non-suitable for non-standard installations.

Depends on: https://reviews.llvm.org/D24351 https://reviews.llvm.org/D24354

[whitequark]

Please just fold these three patches into one.

[mgorny]

Well, three commits look nicer in git but I guess doing them in phabricator is hateful. I'll do that.

[whitequark]

I don't see any point in having them separately in git either, it is just confusing since you are making logically one change to one component...

cmake/modules/AddOCaml.cmake

if( NOT (ext STREQUAL ".cmo" OR
              ext STREQUAL CMAKE_C_OUTPUT_EXTENSION 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()
 
+  set(OCAML_INSTALL_PATH "${OCAML_STDLIB_PATH}/llvm"
+      CACHE STRING "Install directory for LLVM Ocaml libraries")
+
   install(FILES ${install_files}
-          DESTINATION "${OCAML_STDLIB_PATH}/llvm")
+          DESTINATION "${OCAML_INSTALL_PATH}")
   install(FILES ${install_shlibs}
           PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE
                       GROUP_READ GROUP_EXECUTE
                       WORLD_READ WORLD_EXECUTE
-          DESTINATION "${OCAML_STDLIB_PATH}/llvm")
+          DESTINATION "${OCAML_INSTALL_PATH}")
 
   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")