Match BUILD_SHARED_LIBS for add_library targets

[c0ffeeartc]

Fixes linker error when linking libcocos2d.so with option(BUILD_SHARED_LIBS ON)

[zilongshanren]

thanks @c0ffeeartc
Should we change other settings in order to link the libcocos2d.so?

[c0ffeeartc]

For my system only these fixes were required.

here are options that I had to change temporarily for to compile and link libcocos2d.so:

set(USE_PREBUILT_LIBS_DEFAULT OFF)
if(MINGW)
  set(USE_PREBUILT_LIBS_DEFAULT OFF)
endif()

set(BUILD_CPP_TESTS_DEFAULT OFF)
set(BUILD_LUA_LIBS_DEFAULT  OFF)
set(BUILD_LUA_TESTS_DEFAULT OFF)
set(BUILD_JS_LIBS_DEFAULT   OFF)
set(BUILD_JS_TESTS_DEFAULT  OFF)

option(USE_CHIPMUNK "Use chipmunk for physics library" OFF)
option(USE_BULLET "Use bullet for physics3d library" OFF)
option(BUILD_SHARED_LIBS "Build shared libraries" ON)

Building with chipmunk, bullet, lua or js may bring other issues. But with above options + fix libcocos2d.so built fine

[zilongshanren]

@c0ffeeartc maybe we should support building shared libraries for all projects on Linux platform.
But now this feature request doesn't have such high priority.

So I will just leave your PR here. Thanks.