cmake: fix supporting of legacy flags

cmake/OpenCVCompilerOptimizations.cmake

@@ -56,7 +56,7 @@ macro(ocv_optimization_process_obsolete_option legacy_flag OPT legacy_warn)
       message(STATUS "         Behaviour of this option is not backward compatible")
       message(STATUS "         Refer to 'CPU_BASELINE'/'CPU_DISPATCH' CMake options documentation")
     endif()
-    if("${legacy_flag}")
+    if("${${legacy_flag}}")
       if(NOT ";${CPU_BASELINE_REQUIRE};" MATCHES ";${OPT};")
         set(CPU_BASELINE_REQUIRE "${CPU_BASELINE_REQUIRE};${OPT}" CACHE STRING "${HELP_CPU_BASELINE_REQUIRE}" FORCE)
       endif()