Enable toolchain transitions.

See bazelbuild/bazel#11584 and
https://github.com/bazelbuild/proposals/blob/master/designs/2020-02-07-toolchain-transition-migration.md.

WORKSPACE

@@ -32,7 +32,7 @@ load("@bazel_skylib//lib:versions.bzl", "versions")
 
 # Note that the versions library only works within a WORKSPACE file, see
 # https://github.com/bazelbuild/bazel/issues/8305.
-versions.check("3.0.0")
+versions.check("3.4.0")
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive", "http_file")
 

elisp/defs.bzl

@@ -289,6 +289,7 @@ also load each other.  However, it’s often preferable to only have one
 ensure that files get only loaded in their byte-compiled form.""",
     provides = [EmacsLispInfo],
     toolchains = [_TOOLCHAIN_TYPE],
+    incompatible_use_toolchain_transition = True,
     implementation = _elisp_library_impl,
 )
 
@@ -361,6 +362,7 @@ in batch mode.""",
         "@bazel_tools//tools/cpp:toolchain_type",
         _TOOLCHAIN_TYPE,
     ],
+    incompatible_use_toolchain_transition = True,
     implementation = _elisp_binary_impl,
 )
 
@@ -449,6 +451,7 @@ normally pass, but don’t work under coverage for some reason.""",
         "@bazel_tools//tools/cpp:toolchain_type",
         _TOOLCHAIN_TYPE,
     ],
+    incompatible_use_toolchain_transition = True,
     implementation = _elisp_test_impl,
 )
 

emacs/defs.bzl

@@ -96,6 +96,7 @@ The resulting executable can be used to run the compiled Emacs.""",
     executable = True,
     fragments = ["cpp"],
     toolchains = ["@bazel_tools//tools/cpp:toolchain_type"],
+    incompatible_use_toolchain_transition = True,
     implementation = _emacs_binary_impl,
 )