cmake: Use if(... IN_LIST ...) command

Available in CMake 3.3+.

cmake/CheckStringOptionValue.cmake

@@ -1,11 +1,9 @@
 function(check_string_option_value option)
   get_property(expected_values CACHE ${option} PROPERTY STRINGS)
   if(expected_values)
-    foreach(value IN LISTS expected_values)
-      if(value STREQUAL "${${option}}")
-        return()
-      endif()
-    endforeach()
+    if(${option} IN_LIST expected_values)
+      return()
+    endif()
     message(FATAL_ERROR "${option} value is \"${${option}}\", but must be one of ${expected_values}.")
   endif()
   message(AUTHOR_WARNING "The STRINGS property must be set before invoking `check_string_option_value' function.")