Change some USE_ITHREADS to USE_THREADS

These are where the underlying implementation should not matter.

locale.c

@@ -359,7 +359,7 @@ static int debug_initialization = 0;
 /* Automatically include the caller's file, and line number in debugging output;
  * and the errno (and/or extended errno) if non-zero.  On threaded perls add
  * the aTHX too. */
-#  if defined(USE_ITHREADS) && ! defined(NO_LOCALE_THREADS)
+#  if defined(USE_THREADS) && ! defined(NO_LOCALE_THREADS)
 #    define DEBUG_PRE_STMTS                                                 \
         DEBUG_ERRNO;                                                        \
         PerlIO_printf(Perl_debug_log, "\n%s: %" LINE_Tf ": 0x%p%s: ",       \

makedef.pl

@@ -120,15 +120,15 @@ BEGIN
 # perl.h logic duplication begins
 
 
-if ($define{USE_ITHREADS}) {
+if ($define{USE_THREADS}) {
     if (!$define{MULTIPLICITY}) {
         $define{MULTIPLICITY} = 1;
     }
 }
 
 $define{MULTIPLICITY} ||= $define{PERL_IMPLICIT_CONTEXT} ;
 
-if ($define{USE_ITHREADS} && ! $define{WIN32}) {
+if ($define{USE_THREADS} && ! $define{WIN32}) {
     $define{USE_REENTRANT_API} = 1;
 }
 
@@ -149,7 +149,7 @@ BEGIN
 
 # https://en.wikipedia.org/wiki/Microsoft_Visual_C%2B%2B#Internal_version_numbering
 my $cctype = $ARGS{CCTYPE} =~ s/MSVC//r;
-if (   $define{USE_ITHREADS}
+if (   $define{USE_THREADS}
     && $define{USE_LOCALE}
     && ! $define{NO_LOCALE_THREADS})
 {

perl.h

@@ -120,7 +120,7 @@ no normal arguments, and used by L</C<comma_pDEPTH>> itself.
 /* See L<perlguts/"The Perl API"> for detailed notes on
  * MULTIPLICITY and PERL_IMPLICIT_SYS */
 
-#ifdef USE_ITHREADS
+#ifdef USE_THREADS
 #  if !defined(MULTIPLICITY)
 #    define MULTIPLICITY
 #  endif
@@ -1226,7 +1226,7 @@ typedef enum {
  * The defines from here to the following ===== line are unfortunately
  * duplicated in makedef.pl, and changes here MUST also be made there */
 
-#  if defined(USE_ITHREADS) && ! defined(NO_LOCALE_THREADS)
+#  if defined(USE_THREADS) && ! defined(NO_LOCALE_THREADS)
 #    define USE_LOCALE_THREADS
 #  endif
 
@@ -5223,7 +5223,7 @@ typedef Sighandler_t Sigsave_t;
 
 #if defined(USE_PERLIO)
 EXTERN_C void PerlIO_teardown(void);
-# ifdef USE_ITHREADS
+# ifdef USE_THREADS
 #  define PERLIO_INIT MUTEX_INIT(&PL_perlio_mutex)
 #  define PERLIO_TERM 				\
         STMT_START {				\
@@ -6911,7 +6911,7 @@ typedef struct am_table_short AMTS;
 
 #define PERLDB_LINE_OR_SAVESRC (PL_perldb & (PERLDBf_LINE | PERLDBf_SAVESRC))
 
-#ifdef USE_ITHREADS
+#ifdef USE_THREADS
 #  define KEYWORD_PLUGIN_MUTEX_INIT    MUTEX_INIT(&PL_keyword_plugin_mutex)
 #  define KEYWORD_PLUGIN_MUTEX_LOCK    MUTEX_LOCK(&PL_keyword_plugin_mutex)
 #  define KEYWORD_PLUGIN_MUTEX_UNLOCK  MUTEX_UNLOCK(&PL_keyword_plugin_mutex)