Target macOS 10.12 when building LLVM

BUILD.gn

@@ -15,6 +15,7 @@
 import("//build/config/c++/c++.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/mips.gni")
+import("//build/config/sysroot.gni")
 import("src/Reactor/reactor.gni")
 
 config("swiftshader_config") {
@@ -68,6 +69,14 @@ config("swiftshader_config") {
       "-fno-operator-names",
     ]
     cflags_cc = [ "-std=c++17" ]
+    # On macOS, some C++17 features used by LLVM require version 10.12.
+    if (is_mac && supports_llvm) {
+      cflags += [
+        "-isysroot",
+        rebase_path(sysroot, root_build_dir),
+        "-mmacosx-version-min=10.12.0",
+      ]
+    }
 
     defines += [
       "__STDC_CONSTANT_MACROS",

src/Reactor/reactor.gni

@@ -9,9 +9,16 @@ import("../swiftshader.gni")
 import("//build_overrides/build.gni")
 
 declare_args() {
-  # Subzero doesn't support ARM64, MIPS64, and PPC64.
+  # Subzero doesn't support ARM64, MIPS64, and PPC64 (only x86 and ARMv7a).
   supports_subzero = current_cpu != "arm64" && current_cpu != "mips64el" && current_cpu != "ppc64"
-  supports_llvm = is_linux || is_chromeos || is_fuchsia || is_win || is_android || is_mac
+}
+
+declare_args() {
+  supports_llvm = is_linux || is_chromeos || is_fuchsia || is_win || is_android
+    # LLVM uses C++17 features which require macOS 10.12, while Chrome's minimum platform for x86 is 10.11.
+    # Don't build LLVM on Mac, unless we have to. This only happens on ARM64 devices, which launched with 11.0.
+    # TODO(b/174843857): Remove check for !supports_subzero once Chrome supports macOS 10.12
+    || (is_mac && !supports_subzero)
 }
 
 declare_args() {