Backends: Don't forward declare std::thread

Turns out it's undefined behavior, for reference: http://www.gotw.ca/gotw/034.htm

On FreeBSD std::thread is forward declared by at least one other standard header we use, but not on GhostBSD.
Both make use of a nested inline namespace for library versioning, resulting in std::__1::thread.

Basically, there is no portable way to achieve forward declarations for standard containers.
The good news is that including the proper header(s) shouldn't slow down compilation at all.

Author: davidebeatrici

src/backends/ALSA/Flux.cpp

@@ -11,7 +11,6 @@
 #include <cstddef>
 #include <cstring>
 #include <functional>
-#include <thread>
 #include <vector>
 
 #include <alsa/asoundlib.h>

src/backends/ALSA/Flux.hpp

@@ -12,10 +12,7 @@
 #include <atomic>
 #include <cstdint>
 #include <memory>
-
-namespace std {
-class thread;
-}
+#include <thread>
 
 typedef CrossAudio_ErrorCode ErrorCode;
 

src/backends/OSS/Flux.cpp

@@ -10,7 +10,6 @@
 #include <cstdint>
 #include <cstring>
 #include <functional>
-#include <thread>
 #include <type_traits>
 #include <vector>
 

src/backends/OSS/Flux.hpp

@@ -13,12 +13,7 @@
 
 #include <atomic>
 #include <memory>
-
-#ifdef __linux__
-namespace std {
-class thread;
-}
-#endif
+#include <thread>
 
 typedef CrossAudio_ErrorCode ErrorCode;
 

src/backends/Sndio/Flux.cpp

@@ -12,7 +12,6 @@
 #include <cstdint>
 #include <cstring>
 #include <functional>
-#include <thread>
 #include <vector>
 
 #include <poll.h>

src/backends/Sndio/Flux.hpp

@@ -12,12 +12,7 @@
 #include <atomic>
 #include <cstdint>
 #include <memory>
-
-#ifdef __linux__
-namespace std {
-class thread;
-}
-#endif
+#include <thread>
 
 typedef CrossAudio_ErrorCode ErrorCode;
 

src/backends/WASAPI/Flux.cpp

@@ -11,7 +11,6 @@
 #include <bit>
 #include <cstring>
 #include <functional>
-#include <thread>
 
 #include <Windows.h>
 

src/backends/WASAPI/Flux.hpp

@@ -8,14 +8,11 @@
 
 #include <atomic>
 #include <memory>
+#include <thread>
 
 #include "crossaudio/ErrorCode.h"
 #include "crossaudio/Flux.h"
 
-namespace std {
-class thread;
-}
-
 typedef CrossAudio_ErrorCode ErrorCode;
 
 typedef CrossAudio_FluxConfig FluxConfig;