Fix Mac host builds in release configuration.

Change-Id: I5a7f28fbad462ac940b8df4fbe465138198eedfd
diff --git a/BUILD.gn b/BUILD.gn
index 64b7770..e4218a8 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -12,6 +12,10 @@
     cflags += [
       "-Wno-unused-value",
 
+      # Harfbuzz defines variables that are only used in certain assertions that
+      # are stripped away in some build variants.
+      "-Wno-unused-variable",
+
       "-Wno-tautological-compare",
 
       "-Wno-parentheses-equality",