Support building dll on Windows (#143)

CMakeLists.txt

@@ -43,6 +43,12 @@ if(DEFINED ANDROID_ABI)
   set(SHERPA_NCNN_ENABLE_JNI ON CACHE BOOL "" FORCE)
 endif()
 
+# See
+# https://stackoverflow.com/questions/33062728/cmake-link-shared-library-on-windows
+if(BUILD_SHARED_LIBS AND MSVC)
+  set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS ON)
+endif()
+
 message(STATUS "BUILD_SHARED_LIBS ${BUILD_SHARED_LIBS}")
 message(STATUS "SHERPA_NCNN_ENABLE_PYTHON ${SHERPA_NCNN_ENABLE_PYTHON}")
 message(STATUS "SHERPA_NCNN_ENABLE_PORTAUDIO ${SHERPA_NCNN_ENABLE_PORTAUDIO}")