Consistently use USE_DSTRING instead of USE_STD_STRING

`USE_STD_STRING` is to be set on the build command line, but internally
we always check for `USE_DSTRING`.

Author: tautschnig

src/util/json.h

@@ -274,7 +274,7 @@ class json_stringt:public jsont
   {
   }
 
-#ifndef USE_STD_STRING
+#ifdef USE_DSTRING
   explicit json_stringt(const irep_idt &_value)
     : jsont(kindt::J_STRING, id2string(_value))
   {

unit/util/irep.cpp

@@ -22,7 +22,7 @@ SCENARIO("irept_memory", "[core][utils][irept]")
       const std::size_t ref_count_size = 0;
 #endif
 
-#ifndef USE_STD_STRING
+#ifdef USE_DSTRING
       const std::size_t data_size = sizeof(dstringt);
       REQUIRE(sizeof(dstringt) == sizeof(unsigned));
 #else