Another attempt to reduce flaky builds on GitHub.

See bazelbuild/bazel#14113 (comment).

build.py

@@ -159,6 +159,11 @@ def _bazel(command: str, targets: Iterable[str], *,
     args.extend(targets)
     env = dict(os.environ)
     attempts = 1
+    if kernel == 'Darwin':
+        # We don’t need XCode, and using the Unix toolchain tends to be less
+        # flaky.  See
+        # https://github.com/bazelbuild/bazel/issues/14113#issuecomment-999794586.
+        env['BAZEL_USE_CPP_ONLY_TOOLCHAIN'] = '1'
     if env.get('CI') == 'true':
         # Hacks so that Bazel finds the right binaries on GitHub.  See
         # https://docs.github.com/en/actions/learn-github-actions/environment-variables#default-environment-variables.

emacs/BUILD

@@ -14,6 +14,7 @@
 
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
 load("@bazel_skylib//lib:selects.bzl", "selects")
+load("//elisp:util.bzl", "cc_defaults")
 
 alias(
     name = "emacs",
@@ -204,3 +205,25 @@ constraint_value(
         "@gnu_emacs_27.2//:__pkg__",
     ],
 )
+
+cc_defaults(
+    name = "defaults",
+    copts = [],
+    linkopts = select({
+        "//constraints:macos": [
+            # Override the toolchain’s “-undefined dynamic” option so that the
+            # configure script doesn’t incorrectly detect absent functions as
+            # present.
+            "-undefined",
+            "error",
+        ],
+        "//conditions:default": [],
+    }),
+    visibility = [
+        "@gnu_emacs_26.1//:__pkg__",
+        "@gnu_emacs_26.2//:__pkg__",
+        "@gnu_emacs_26.3//:__pkg__",
+        "@gnu_emacs_27.1//:__pkg__",
+        "@gnu_emacs_27.2//:__pkg__",
+    ],
+)

emacs/defs.bzl

@@ -114,6 +114,10 @@ recommended.""",
             default = "//elisp:wrapper_defaults",
             providers = [CcDefaultInfo],
         ),
+        "_emacs_defaults": attr.label(
+            default = ":defaults",
+            providers = [CcDefaultInfo],
+        ),
     },
     doc = """Builds Emacs from a source repository.
 The resulting executable can be used to run the compiled Emacs.""",
@@ -135,10 +139,11 @@ def _install(ctx, cc_toolchain, source):
     Returns:
       a File representing the Emacs installation directory
     """
+    defaults = ctx.attr._emacs_defaults[CcDefaultInfo]
     feature_configuration = cc_common.configure_features(
         ctx = ctx,
         cc_toolchain = cc_toolchain,
-        requested_features = ctx.features,
+        requested_features = defaults.features + ctx.features,
         # Never instrument Emacs itself for coverage collection.  It doesn’t
         # hurt, but leads to needless reinstall actions when switching between
         # “bazel test” and “bazel coverage”.
@@ -157,14 +162,14 @@ def _install(ctx, cc_toolchain, source):
         feature_configuration = feature_configuration,
         action_name = C_COMPILE_ACTION_NAME,
         variables = vars,
-    ) + fragment.copts + fragment.conlyopts + [
+    ) + fragment.copts + fragment.conlyopts + defaults.copts + [
         "-DNO_SHLWAPI_ISOS",  # Work around https://bugs.gnu.org/48303
     ]
     ldflags = cc_common.get_memory_inefficient_command_line(
         feature_configuration = feature_configuration,
         action_name = CPP_LINK_EXECUTABLE_ACTION_NAME,
         variables = vars,
-    ) + fragment.linkopts
+    ) + fragment.linkopts + defaults.linkopts
 
     # The build process needs to find some common binaries like “make” or the
     # GNU coreutils.