meson.build: remove the libwinpthread-1.dll dependency on Windows

Closes https://github.com/MusicPlayerDaemon/MPD/issues/507

NEWS

@@ -7,6 +7,8 @@ ver 0.21.6 (not yet released)
 * playlist
   - flac: fix use-after-free bug
 * support abstract sockets on Linux
+* Windows
+  - remove the unused libwinpthread-1.dll dependency
 
 ver 0.21.5 (2019/02/22)
 * protocol

src/thread/meson.build

@@ -1,4 +1,11 @@
-threads_dep = dependency('threads')
+if is_windows
+  # avoid the unused libwinpthread-1.dll dependency on Windows; MPD
+  # doesn't use the pthread API on Windows, but this is what Meson
+  # unhelpfully detects for us
+  threads_dep = []
+else
+  threads_dep = dependency('threads')
+endif
 
 conf.set('HAVE_PTHREAD_SETNAME_NP', compiler.has_function('pthread_setname_np', dependencies: threads_dep))