Husshhhh.... The issue is finally resolved...:)
Looks like there is some linker problem with the MinGW 64 bit compiler. The solution is to link the libopencl.a file instead of OpenCL.dll for building 64 bit applications.
While building for 32 bit applications does not require this i.e. you can link with OpenCL.dll.
Now only if somebody can fix this issue which has been open for past 4 years!