Update README.md for GPU_TARGETS (#2423)

README.md

@@ -67,7 +67,7 @@ The following is a list of prerequisites for building MIGraphX.
 3. Build MIGraphX source code:
 
     ```bash
-    rbuild build -d depend -B build
+    rbuild build -d depend -B build -DGPU_TARGETS=$(/opt/rocm/bin/rocminfo | grep -o -m1 'gfx.*')
     ```
 
 Once completed, all prerequisites are in the `depend` folder and MIGraphX is in the `build` directory.
@@ -106,7 +106,7 @@ the folder to `PATH`, or add the option `--prefix /usr/local` in the pip3 comman
 3. Configure CMake. If the prerequisites are installed at the default location `/usr/local`, use:
 
     ```bash
-    CXX=/opt/rocm/llvm/bin/clang++ cmake ..
+    CXX=/opt/rocm/llvm/bin/clang++ cmake .. -DGPU_TARGETS=$(/opt/rocm/bin/rocminfo | grep -o -m1 'gfx.*')
     ```
 
     Otherwise, you need to set `-DCMAKE_PREFIX_PATH=$your_loc` to configure CMake.