[gn build] make WindowsManifestMerger.cpp build fine with sysroot

[thakis]

Summary

This already works in the cmake build.

llvm/utils/gn/build/BUILD.gn

 import("//llvm/utils/gn/build/buildflags.gni")
 import("//llvm/utils/gn/build/mac_sdk.gni")
+import("//llvm/utils/gn/build/sysroot.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 import("//llvm/utils/gn/build/toolchain/target_flags.gni")
 
 declare_args() {
   # Whether to build everything with coverage information.
   # After building with this, run tests and then run
   #    llvm/utils/prepare-code-coverage-artifact.py  \
   #        .../llvm-profdata .../llvm-cov out/gn/profiles/ report/ \
   #        out/gn/bin/llvm-undname ...`
   # to generate a HTML report for the binaries passed in the last line.
   llvm_build_instrumented_coverage = false
 
   # If set, puts relative paths in debug info.
   # Makes the build output independent of the build directory, but makes
   # most debuggers harder to use. See "Getting to local determinism" and
   # "Getting debuggers to work well with locally deterministic builds" in
   # http://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html
   # for more information.
   use_relative_paths_in_debug_info = false
 
   # The version of host gcc. Ignored if is_clang is true.
   gcc_version = 9
-
-  # Path of sysroot to use.
-  sysroot = ""
 }
 
 assert(!llvm_build_instrumented_coverage || is_clang,
        "llvm_build_instrumented_coverage requires clang as host compiler")
 
 config("compiler_defaults") {
   defines = []
 
config("compiler_defaults") {
   if (sysroot != "") {
     assert(current_os != "mac" && current_os != "ios",
            "FIXME: Make sysroot work on darwin")
     if (current_os == "win") {
       assert(is_clang, "sysroot only works with clang-cl as host compiler")
       cflags += [ "/winsysroot" + rebase_path(sysroot, root_build_dir) ]
     } else {
       cflags += [ "--sysroot=" + rebase_path(sysroot, root_build_dir) ]
+      if (is_clang) {
+        cflags += [ "-Wpoison-system-directories"]
+      }
     }
   }
 
   if (use_ubsan) {
     assert(is_clang && current_os == "linux",
            "ubsan only supported on Linux/Clang")
     cflags += [ "-fsanitize=undefined" ]
     ldflags += [ "-fsanitize=undefined" ]

llvm/utils/gn/build/libs/xml/BUILD.gn

 import("//llvm/utils/gn/build/libs/xml/enable.gni")
 import("//llvm/utils/gn/build/mac_sdk.gni")
+import("//llvm/utils/gn/build/sysroot.gni")
 
 config("xml_config") {
   visibility = [ ":xml" ]
   libs = [ "xml2" ]
   if (host_os == "mac") {
     include_dirs = [ "$mac_sdk_path/usr/include/libxml2" ]
   } else {
-    include_dirs = [ "/usr/include/libxml2" ]
+    include_dirs = [ "$sysroot/usr/include/libxml2" ]
   }
 }
 
 group("xml") {
   if (llvm_enable_libxml2) {
     public_configs = [ ":xml_config" ]
   }
 }

llvm/utils/gn/build/sysroot.gni

+declare_args() {
+  # Path of sysroot to use.
+  sysroot = ""
+}