Use @loader_path instead of $ORIGIN on MacOS (#403)

The dynamic libraries currently produced by rapids-cmake on MacOS for cython modules don't work, because the wrong "ORIGIN" marker is used. The typical suggestion appears to be to use `@loader_path` in place of `$ORIGIN` on MacOS, although there is one more option that I can't currently recall.

Please excuse my ignorance around your processes. Please feel free to make/request changes.

Authors:
  - Manolis Papadakis (https://github.com/manopapad)
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - Robert Maynard (https://github.com/robertmaynard)
  - Bradley Dice (https://github.com/bdice)

URL: #403

example/CMakeLists.txt

@@ -98,8 +98,8 @@ rapids_cpm_find(fmt 7.1.3
 add_library(example SHARED source.cu)
 
 set_target_properties(example
-    PROPERTIES BUILD_RPATH                         "\$ORIGIN"
-               INSTALL_RPATH                       "\$ORIGIN"
+    PROPERTIES BUILD_RPATH                         "$ORIGIN"
+               INSTALL_RPATH                       "$ORIGIN"
                # set target compile options
                CXX_STANDARD                        17
                CXX_STANDARD_REQUIRED               ON

rapids-cmake/cython/add_rpath_entries.cmake

@@ -72,14 +72,19 @@ function(rapids_cython_add_rpath_entries)
     list(APPEND cleaned_paths "${path}")
   endforeach()
 
+  if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+    set(platform_rpath_origin "@loader_path")
+  else()
+    set(platform_rpath_origin "$ORIGIN")
+  endif()
   get_property(targets GLOBAL PROPERTY "rapids_cython_associations_${_RAPIDS_CYTHON_TARGET}")
   foreach(target IN LISTS targets)
     # Compute the path relative to the current target.
     set(target_paths)
     get_target_property(target_source_dir ${target} SOURCE_DIR)
     foreach(target_path IN LISTS cleaned_paths)
       cmake_path(RELATIVE_PATH target_path BASE_DIRECTORY "${target_source_dir}")
-      list(APPEND target_paths "$ORIGIN/${target_path}")
+      list(APPEND target_paths "${platform_rpath_origin}/${target_path}")
     endforeach()
     list(JOIN target_paths ";" target_paths)
 

rapids-cmake/cython/create_modules.cmake

@@ -127,7 +127,12 @@ function(rapids_cython_create_modules)
     install(TARGETS ${cython_module} DESTINATION ${_RAPIDS_CYTHON_INSTALL_DIR})
 
     # Default the INSTALL_RPATH for all modules to $ORIGIN.
-    set_target_properties(${cython_module} PROPERTIES INSTALL_RPATH "\$ORIGIN")
+    if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+      set(platform_rpath_origin "@loader_path")
+    else()
+      set(platform_rpath_origin "$ORIGIN")
+    endif()
+    set_target_properties(${cython_module} PROPERTIES INSTALL_RPATH "${platform_rpath_origin}")
 
     # Store any provided associated targets in a global list
     foreach(associated_target IN LISTS _RAPIDS_CYTHON_ASSOCIATED_TARGETS)