Make libdawn a shared library, add a DAWN_EXPORT macro

src/include/dawn/dawn_export.h

@@ -0,0 +1,32 @@
+// Copyright 2018 The Dawn Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef DAWN_EXPORT_H_
+#define DAWN_EXPORT_H_
+
+#if defined(WIN32)
+#    if defined(DAWN_IMPLEMENTATION)
+#        define DAWN_EXPORT __declspec(dllexport)
+#    else
+#        define DAWN_EXPORT __declspec(dllimport)
+#    endif
+#else
+#    if defined(DAWN_IMPLEMENTATION)
+#        define DAWN_EXPORT __attribute__((visibility("default")))
+#    else
+#        define DAWN_EXPORT
+#    endif
+#endif
+
+#endif  // DAWN_EXPORT_H_