OpenCL compute added for Inter Prediction and Model RD

NOTE :
1) Added the following configure options
    --enable-opencl     - To enable OpenCL compute
    --opencl-lib=PATH   - Name of the OpenCL library in full path for linking
                          (To be used only when OpenCL is enabled)

Example : --enable-opencl --opencl-lib=C:/Windows/System32/OpenCL.dll

2) Right now the OpenCL kernel name and path is hardcoded in vp9_opencl.c
   Will be fixed soon

3) This commit only focuses on functionality of the OpenCL code
   OpenCL Performance optimizations will be taken up later

4) Multithreading does not work with GPU compute.
   Will be fixed soon