Update for XCode 11 include system

With XCode 11 there is no more /usr/include directory. This
does not affect the XCode compilers, but does make goma compiler
break. This finds the system includes the same way that the iOS
builds currently do.

The gn_to_bp.py tool runs gn with target_os == mac, but host_os != mac.
In that case, the xcrun tool can not run, so make xcode_sysroot = "". This will
allow the xcode compilers to work when using the android.bp to build things.

TESTED:
builds on mac with and without goma

Change-Id: I9de6797c32760c59e62fe5ac505a3404e5eaf8e9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/249537
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>

Author: herbderby

gn/BUILD.gn

@@ -12,9 +12,10 @@ declare_args() {
   use_PIC = true
 
   malloc = ""
+  xcode_sysroot = ""
 }
 
-if (is_ios) {
+if (is_ios && xcode_sysroot == "") {
   if (is_tvos) {
     sdk = "appletvos"
     if (target_cpu == "x86" || target_cpu == "x64") {
@@ -26,7 +27,16 @@ if (is_ios) {
       sdk = "iphonesimulator"
     }
   }
-  ios_sysroot = exec_script("find_ios_sysroot.py", [ sdk ], "trim string")
+  xcode_sysroot = exec_script("find_xcode_sysroot.py", [ sdk ], "trim string")
+}
+
+# If building for mac on a mac then lookup all the system includes so that goma and the clang
+# shipped with chrome can find them. When the gn_to_bp.py tool is run, then the host_os != mac.
+# In this case leave the xcode_sysroot empty, and the cc/c++ that come with XCode will be able to
+# find needed include paths.
+if (is_mac && host_os == "mac" && xcode_sysroot == "") {
+  xcode_sysroot =
+      exec_script("find_xcode_sysroot.py", [ "macosx" ], "trim string")
 }
 
 config("default") {
@@ -157,20 +167,20 @@ config("default") {
     }
     asmflags += [
       "-isysroot",
-      ios_sysroot,
+      xcode_sysroot,
       "-arch",
       _target,
     ]
     cflags += [
       "-isysroot",
-      ios_sysroot,
+      xcode_sysroot,
       "-arch",
       _target,
     ]
     cflags_cc += [ "-stdlib=libc++" ]
     ldflags += [
       "-isysroot",
-      ios_sysroot,
+      xcode_sysroot,
       "-arch",
       _target,
       "-stdlib=libc++",
@@ -184,7 +194,25 @@ config("default") {
       defines += [ "_GLIBCXX_DEBUG" ]
     }
   }
+
   if (is_mac) {
+    # If there was a xcode_sysroot set in args or calculated then use it, else don't set anything
+    # because the XCode cc/c++ already know all this stuff.
+    if (xcode_sysroot != "") {
+      asmflags += [
+        "-isysroot",
+        xcode_sysroot,
+      ]
+      cflags += [
+        "-isysroot",
+        xcode_sysroot,
+      ]
+      ldflags += [
+        "-isysroot",
+        xcode_sysroot,
+      ]
+    }
+
     # Disable linker warnings.  They're usually just annoyances like,
     #   ld: warning: text-based stub file
     #     /System/Library/Frameworks/foo.framework/foo.tbd and library file