[docs] Update GPU-Tutorial.rst (#1037)

* Update GPU-Tutorial.rst

* Update Installation-Guide.rst

* Update GPU-Tutorial.rst

docs/GPU-Tutorial.rst

@@ -61,6 +61,8 @@ Now we are ready to checkout LightGBM and compile it with GPU support:
     cd LightGBM
     mkdir build ; cd build
     cmake -DUSE_GPU=1 .. 
+    # if you have installed the NVIDIA OpenGL, please using following instead
+    # sudo cmake -DUSE_GPU=1 -DOpenCL_LIBRARY=/usr/local/cuda/lib64/libOpenCL.so -OpenCL_INCLUDE_DIR=/usr/local/cuda/include/ ..
     make -j$(nproc)
     cd ..
 

docs/Installation-Guide.rst

@@ -227,6 +227,8 @@ To build LightGBM GPU version, run the following commands:
   git clone --recursive https://github.com/Microsoft/LightGBM ; cd LightGBM
   mkdir build ; cd build
   cmake -DUSE_GPU=1 ..
+  # if you have installed the NVIDIA OpenGL, please using following instead
+  # sudo cmake -DUSE_GPU=1 -DOpenCL_LIBRARY=/usr/local/cuda/lib64/libOpenCL.so -OpenCL_INCLUDE_DIR=/usr/local/cuda/include/ ..
   make -j4
 
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