Split mojo_system dylib into public and private

mojo/mojo_public.gypi

 {
   'targets': [
     {
+      'target_name': 'mojo_system',
+      'type': 'shared_library',
+      'defines': [
+        'MOJO_SYSTEM_IMPLEMENTATION',
+      ],
+      'include_dirs': [
+        '..',
+      ],
+      'direct_dependent_settings': {
+        'include_dirs': [
+          '..',
+        ],
+      },
+      'sources': [
+        'public/system/core.h',
+        'public/system/core_cpp.h',
+        'public/system/core_private.cc',
+        'public/system/core_private.h',
+        'public/system/macros.h',
+        'public/system/system_export.h',
+      ],
+    },
+    {
       'target_name': 'mojo_public_test_support',
       'type': 'static_library',
       'dependencies': [
         '../base/base.gyp:base',
         '../testing/gtest.gyp:gtest',
         'mojo_system',
       ],
       'sources': [
         'public/tests/simple_bindings_support.cc',
         'public/tests/simple_bindings_support.h',
@@ skipping 87 matching lines @@
       ],
       'dependencies': [
         '../testing/gtest.gyp:gtest',
         'mojo_public_test_support',
         'mojo_run_all_unittests',
         'sample_service',
       ],
     },
   ],
 }