Create a new rule, cc_flags_supplier, which provides the CC_FLAGS Make variable. PiperOrigin-RevId: 228178590

diff --git a/tools/cpp/cc_flags_supplier_lib.bzl b/tools/cpp/cc_flags_supplier_lib.bzl
new file mode 100644
index 0000000..a179fbf
--- /dev/null
+++ b/tools/cpp/cc_flags_supplier_lib.bzl

@@ -0,0 +1,70 @@
+# Copyright 2018 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Library of functions that provide the CC_FLAGS Make variable."""
+
+# This should match the logic in CcCommon.computeCcFlags:
+def build_cc_flags(ctx, cc_toolchain, action_name):
+    """Determine the value for CC_FLAGS based on the given toolchain."""
+
+    # Get default cc flags from toolchain's make_variables.
+    legacy_cc_flags = cc_common.legacy_cc_flags_make_variable_do_not_use(
+        cc_toolchain = cc_toolchain,
+    )
+
+    # Determine the sysroot flag.
+    sysroot_cc_flags = _from_sysroot(cc_toolchain)
+
+    # Flags from feature config.
+    feature_config_cc_flags = _from_features(ctx, cc_toolchain, action_name)
+
+    # Combine the different sources, but only add the sysroot flag if nothing
+    # else adds sysroot.
+    # If added, it must appear before the feature config flags.
+    cc_flags = []
+    if legacy_cc_flags:
+        cc_flags.append(legacy_cc_flags)
+    if sysroot_cc_flags and not _contains_sysroot(feature_config_cc_flags):
+        cc_flags.append(sysroot_cc_flags)
+    cc_flags.extend(feature_config_cc_flags)
+
+    return " ".join(cc_flags)
+
+def _contains_sysroot(flags):
+    for flag in flags:
+        if "--sysroot=" in flag:
+            return True
+    return False
+
+def _from_sysroot(cc_toolchain):
+    sysroot = cc_toolchain.sysroot
+    if sysroot:
+        return "--sysroot=%s" % sysroot
+    else:
+        return None
+
+def _from_features(ctx, cc_toolchain, action_name):
+    feature_configuration = cc_common.configure_features(
+        cc_toolchain = cc_toolchain,
+        requested_features = ctx.features,
+        unsupported_features = ctx.disabled_features,
+    )
+
+    variables = cc_common.empty_variables()
+
+    cc_flags = cc_common.get_memory_inefficient_command_line(
+        feature_configuration = feature_configuration,
+        action_name = action_name,
+        variables = variables,
+    )
+    return cc_flags