Add external_include_paths feature (#327)

Mirrors this upstream feature
bazelbuild/bazel#12009

Author: keith

crosstool/cc_toolchain_config.bzl

@@ -918,6 +918,32 @@ please file an issue at https://github.com/bazelbuild/apple_support/issues/new
         ],
     )
 
+    external_include_paths_feature = feature(
+        name = "external_include_paths",
+        flag_sets = [
+            flag_set(
+                actions = [
+                    ACTION_NAMES.preprocess_assemble,
+                    ACTION_NAMES.linkstamp_compile,
+                    ACTION_NAMES.c_compile,
+                    ACTION_NAMES.cpp_compile,
+                    ACTION_NAMES.cpp_header_parsing,
+                    ACTION_NAMES.cpp_module_compile,
+                    ACTION_NAMES.clif_match,
+                    ACTION_NAMES.objc_compile,
+                    ACTION_NAMES.objcpp_compile,
+                ],
+                flag_groups = [
+                    flag_group(
+                        flags = ["-isystem", "%{external_include_paths}"],
+                        iterate_over = "external_include_paths",
+                        expand_if_available = "external_include_paths",
+                    ),
+                ],
+            ),
+        ],
+    )
+
     strip_debug_symbols_feature = feature(
         name = "strip_debug_symbols",
         flag_sets = [
@@ -2665,6 +2691,7 @@ please file an issue at https://github.com/bazelbuild/apple_support/issues/new
         treat_warnings_as_errors_feature,
         no_warn_duplicate_libraries_feature,
         layering_check_feature,
+        external_include_paths_feature,
     ]
 
     if (ctx.attr.cpu == "darwin_x86_64" or