GN: fix mac build again

Hoping to land these using the other GN bots as trybots.

Don't know what magic was letting us get to webp's headers yesterday on Linux.  Might have been using /usr/include's ?

The other change is the difference between some setups using #define SK_BUILD_FOR_MAC and others #define SK_BUILD_FOR_MAC 1.  We want either to mean "we're Mac".

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2190713004

Review-Url: https://codereview.chromium.org/2190713004
diff --git a/third_party/libwebp/BUILD.gn b/third_party/libwebp/BUILD.gn
index 99bf546..1ad734f 100644
--- a/third_party/libwebp/BUILD.gn
+++ b/third_party/libwebp/BUILD.gn
@@ -9,7 +9,7 @@
 import("../third_party.gni")
 
 third_party("libwebp") {
-  public_include_dirs = [ "../externals/libwebp" ]
+  public_include_dirs = [ "../externals/libwebp/src" ]
 
   sources = [
     "../externals/libwebp/src/dec/alpha.c",
diff --git a/tools/flags/SkCommandLineFlags.cpp b/tools/flags/SkCommandLineFlags.cpp
index 9205f47..fd7c8ff 100644
--- a/tools/flags/SkCommandLineFlags.cpp
+++ b/tools/flags/SkCommandLineFlags.cpp
@@ -335,7 +335,7 @@
                 flag = flag->next();
             }
             if (!flagMatched) {
-#if SK_BUILD_FOR_MAC
+#if defined(SK_BUILD_FOR_MAC)
                 if (SkStrStartsWith(argv[i], "NSDocumentRevisions")
                         || SkStrStartsWith(argv[i], "-NSDocumentRevisions")) {
                     i++;  // skip YES