fix: use CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR (#8477)

Co-authored-by: Carlos O'Ryan <coryan@google.com>

Author: martinzink

CMakeLists.txt

@@ -300,9 +300,12 @@ function (google_cloud_cpp_enable_features)
                 add_subdirectory(google/cloud/storage)
             endif ()
         else ()
-            if (NOT IS_DIRECTORY "${CMAKE_SOURCE_DIR}/google/cloud/${feature}"
-                OR NOT EXISTS
-                   "${CMAKE_SOURCE_DIR}/google/cloud/${feature}/CMakeLists.txt")
+            if (NOT IS_DIRECTORY
+                "${CMAKE_CURRENT_SOURCE_DIR}/google/cloud/${feature}"
+                OR NOT
+                   EXISTS
+                   "${CMAKE_CURRENT_SOURCE_DIR}/google/cloud/${feature}/CMakeLists.txt"
+            )
                 message(
                     WARNING
                         "${feature} is not a valid feature in google-cloud-cpp, ignored"
@@ -312,15 +315,17 @@ function (google_cloud_cpp_enable_features)
             add_subdirectory(google/cloud/${feature})
             if (GOOGLE_CLOUD_CPP_ENABLE_EXAMPLES
                 AND IS_DIRECTORY
-                    "${CMAKE_SOURCE_DIR}/google/cloud/${feature}/samples")
+                    "${CMAKE_CURRENT_SOURCE_DIR}/google/cloud/${feature}/samples"
+            )
                 add_subdirectory(google/cloud/${feature}/samples)
             endif ()
             # Older libraries used examples/ to host their examples. The
             # directory name cannot be easily changed as cloud.google.com
             # references these files explicitly.
             if (GOOGLE_CLOUD_CPP_ENABLE_EXAMPLES
                 AND IS_DIRECTORY
-                    "${CMAKE_SOURCE_DIR}/google/cloud/${feature}/examples")
+                    "${CMAKE_CURRENT_SOURCE_DIR}/google/cloud/${feature}/examples"
+            )
                 add_subdirectory(google/cloud/${feature}/examples)
             endif ()
         endif ()