Mark C API as dllexports on Windows (#2342)

On Windows, symbols have to be explicitly exported to make them visible/accessible in a shared library. Binaryen.dll currently doesn't export any symbols as a result.

Marking all exported methods as `BINARYEN_API` that is defined as `__declspec(dllexport)` on Windows, unless building a static library.

Author: MichalStrehovsky

CMakeLists.txt

@@ -192,6 +192,7 @@ SET(binaryen_SOURCES
 )
 IF(BUILD_STATIC_LIB)
   ADD_LIBRARY(binaryen STATIC ${binaryen_SOURCES})
+  ADD_DEFINITIONS(-DBUILD_STATIC_LIBRARY)
 ELSE()
   ADD_LIBRARY(binaryen SHARED ${binaryen_SOURCES})
 ENDIF()