Merge pull request #20960 from alalek:fix_icc_support_3.4

cmake/OpenCVCompilerOptimizations.cmake

@@ -238,7 +238,7 @@ if(X86 OR X86_64)
     ocv_intel_compiler_optimization_option(FP16 "-mavx" "/arch:AVX")
     ocv_intel_compiler_optimization_option(AVX "-mavx" "/arch:AVX")
     ocv_intel_compiler_optimization_option(FMA3 "" "")
-    ocv_intel_compiler_optimization_option(POPCNT "" "")
+    ocv_intel_compiler_optimization_option(POPCNT "-mpopcnt" "")  # -mpopcnt is available since ICC 19.0.0
     ocv_intel_compiler_optimization_option(SSE4_2 "-msse4.2" "/arch:SSE4.2")
     ocv_intel_compiler_optimization_option(SSE4_1 "-msse4.1" "/arch:SSE4.1")
     ocv_intel_compiler_optimization_option(SSE3 "-msse3" "/arch:SSE3")

modules/core/include/opencv2/core/cvdef.h

@@ -589,6 +589,8 @@ Cv64suf;
 #  elif __cplusplus >= 201703L
 //   available when compiler is C++17 compliant
 #    define CV_NODISCARD_STD [[nodiscard]]
+#  elif defined(__INTEL_COMPILER)
+     // see above, available when C++17 is enabled
 #  elif defined(_MSC_VER) && _MSC_VER >= 1911 && _MSVC_LANG >= 201703L
 //   available with VS2017 v15.3+ with /std:c++17 or higher; works on functions and classes
 #    define CV_NODISCARD_STD [[nodiscard]]