config: Adds configuration repository rules.

This allows the user to inject a new toolchain configuration adjusted
the requirements for each user.

config/BUILD.bazel

diff --git a/config/BUILD.bazel b/config/BUILD.bazel
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/config/BUILD.bazel

config/rules_cc_toolchain_config.BUILD

diff --git a/config/rules_cc_toolchain_config.BUILD b/config/rules_cc_toolchain_config.BUILD
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/config/rules_cc_toolchain_config.BUILD

config/rules_cc_toolchain_config_repository.bzl

diff --git a/config/rules_cc_toolchain_config_repository.bzl b/config/rules_cc_toolchain_config_repository.bzl
new file mode 100644
index 0000000..8eb102e
--- /dev/null
+++ b/config/rules_cc_toolchain_config_repository.bzl
@@ -0,0 +1,12 @@
+def _rules_cc_toolchain_config_impl(repository_ctx):
+    repository_ctx.symlink(repository_ctx.attr.build_file, "BUILD")
+
+rules_cc_toolchain_config = repository_rule(
+    _rules_cc_toolchain_config_impl,
+    attrs = {
+        "build_file": attr.label(
+            allow_single_file = True,
+            default = "@rules_cc_toolchain//config:rules_cc_toolchain_config.BUILD",
+        ),
+    },
+)