Make the Mac build work.

tools/gn/secondary/build/config/BUILDCONFIG.gn

 # =============================================================================
 # BUILD FLAGS
 # =============================================================================
 #
 # This block lists input arguments to the build, along with their default
 # values. GN requires listing them explicitly so it can validate input and have
 # a central place to manage the build flags.
 #
 # If a value is specified on the command line, it will overwrite the defaults
 # given here, otherwise the default will be injected into the root scope.
@@ skipping 157 matching lines @@
 }
 
 if (is_debug) {
   native_compiler_configs += "//build/config:debug"
 } else {
   native_compiler_configs += "//build/config:release"
 }
 
 set_defaults("executable") {
   configs = native_compiler_configs
+  if (is_mac) {
+    configs += "//build/config/mac:mac_dynamic_flags"
+  }
 }
 
 set_defaults("static_library") {
   configs = native_compiler_configs
 }
 
 set_defaults("shared_library") {
   configs = native_compiler_configs
+  if (is_mac) {
+    configs += "//build/config/mac:mac_dynamic_flags"
+  }
 }
 
 # ==============================================================================
 # TOOLCHAIN SETUP
 # ==============================================================================
 
 if (is_win) {
   set_default_toolchain("//build/config/win:32")
 } else if (is_mac) {
   set_default_toolchain("//build/config/mac:clang")
 }