Don't define _POSIX_THREADS unless threads are enabled. (WebAssembly#356)

* Don't define `_POSIX_THREADS` unless threads are enabled.

Fixes WebAssembly#355.

* Remove `_POSIX_THREADS` from predefined-macros.txt.

Author: Dan Gohman

expected/wasm32-wasi/single/predefined-macros.txt

@@ -5125,7 +5125,6 @@
 #define _POSIX_STREAM_MAX 8
 #define _POSIX_SYMLINK_MAX 255
 #define _POSIX_SYMLOOP_MAX 8
-#define _POSIX_THREADS _POSIX_VERSION
 #define _POSIX_THREAD_ATTR_STACKADDR _POSIX_VERSION
 #define _POSIX_THREAD_ATTR_STACKSIZE _POSIX_VERSION
 #define _POSIX_THREAD_CPUTIME _POSIX_VERSION

libc-top-half/musl/include/unistd.h

@@ -316,7 +316,9 @@ pid_t gettid(void);
 #endif
 #define _POSIX_VDISABLE         0
 
+#if defined(__wasilibc_unmodified_upstream) || defined(_REENTRANT)
 #define _POSIX_THREADS          _POSIX_VERSION
+#endif
 #define _POSIX_THREAD_PROCESS_SHARED _POSIX_VERSION
 #define _POSIX_THREAD_SAFE_FUNCTIONS _POSIX_VERSION
 #define _POSIX_THREAD_ATTR_STACKADDR _POSIX_VERSION