Move #includes out of extern "C" blocks

Do some include shuffling for `**.h` files within lib, programs, tests, and zlibWrapper.
`lib/legacy` and `lib/deprecated` are untouched.
`#include`s within `extern "C"` blocks in .cpp files are untouched.

todo: shuffling for `xxhash.h`

lib/common/threading.h

@@ -16,10 +16,6 @@
 
 #include "debug.h"
 
-#if defined (__cplusplus)
-extern "C" {
-#endif
-
 #if defined(ZSTD_MULTITHREAD) && defined(_WIN32)
 
 /**
@@ -60,6 +56,11 @@ extern "C" {
 #define ZSTD_pthread_cond_signal(a)     WakeConditionVariable((a))
 #define ZSTD_pthread_cond_broadcast(a)  WakeAllConditionVariable((a))
 
+
+#if defined (__cplusplus)
+extern "C" {
+#endif
+
 /* ZSTD_pthread_create() and ZSTD_pthread_join() */
 typedef HANDLE ZSTD_pthread_t;
 
@@ -73,10 +74,19 @@ int ZSTD_pthread_join(ZSTD_pthread_t thread);
  */
 
 
+#if defined (__cplusplus)
+}
+#endif
+
 #elif defined(ZSTD_MULTITHREAD)    /* posix assumed ; need a better detection method */
 /* ===   POSIX Systems   === */
 #  include <pthread.h>
 
+
+#if defined (__cplusplus)
+extern "C" {
+#endif
+
 #if DEBUGLEVEL < 1
 
 #define ZSTD_pthread_mutex_t            pthread_mutex_t
@@ -123,9 +133,18 @@ int ZSTD_pthread_cond_destroy(ZSTD_pthread_cond_t* cond);
 
 #endif
 
+#if defined (__cplusplus)
+}
+#endif
+
 #else  /* ZSTD_MULTITHREAD not defined */
 /* No multithreading support */
 
+
+#if defined (__cplusplus)
+extern "C" {
+#endif
+
 typedef int ZSTD_pthread_mutex_t;
 #define ZSTD_pthread_mutex_init(a, b)   ((void)(a), (void)(b), 0)
 #define ZSTD_pthread_mutex_destroy(a)   ((void)(a))