Pass through --sysroot to host compiler (#185)

cuda/private/cuda_helper.bzl

@@ -3,6 +3,7 @@
 load("@bazel_skylib//lib:paths.bzl", "paths")
 load("@bazel_skylib//lib:types.bzl", "types")
 load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
+load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain")
 load("//cuda/private:action_names.bzl", "ACTION_NAMES")
 load("//cuda/private:artifact_categories.bzl", "ARTIFACT_CATEGORIES")
 load("//cuda/private:providers.bzl", "ArchSpecInfo", "CudaArchsInfo", "CudaInfo", "Stage2ArchInfo", "cuda_archs")
@@ -242,6 +243,8 @@ def _create_common(ctx):
 
     merged_cc_info = cc_common.merge_cc_infos(cc_infos = [dep[CcInfo] for dep in all_cc_deps])
 
+    cc_toolchain = find_cpp_toolchain(ctx)
+
     # gather include info
     includes = merged_cc_info.compilation_context.includes.to_list()
     system_includes = []
@@ -276,6 +279,8 @@ def _create_common(ctx):
     host_defines = []
     host_local_defines = [i for i in attr.host_local_defines]
     host_compile_flags = attr._default_host_copts[BuildSettingInfo].value + [i for i in attr.host_copts]
+    if cc_toolchain.sysroot:
+        host_compile_flags.append("--sysroot={}".format(cc_toolchain.sysroot))
     host_link_flags = []
     if hasattr(attr, "host_linkopts"):
         host_link_flags.extend([i for i in attr.host_linkopts])