Do not register OpenCL platform if no implementations are available

platforms/opencl/src/OpenCLPlatform.cpp

@@ -141,6 +141,12 @@ bool OpenCLPlatform::isPlatformSupported() {
         return false;
 #endif
 
+    // Make sure at least one OpenCL implementation is installed.
+
+    std::vector<cl::Platform> platforms;
+    cl::Platform::get(&platforms);
+    if (platforms.size() == 0)
+        return false;
     return true;
 }