Make std.Thread.use_pthreads return false in single-threaded builds

Author: castholm

lib/std/Thread.zig

@@ -20,7 +20,7 @@ pub const RwLock = @import("Thread/RwLock.zig");
 pub const Pool = @import("Thread/Pool.zig");
 pub const WaitGroup = @import("Thread/WaitGroup.zig");
 
-pub const use_pthreads = native_os != .windows and native_os != .wasi and builtin.link_libc;
+pub const use_pthreads = native_os != .windows and native_os != .wasi and builtin.link_libc and !builtin.single_threaded;
 
 /// A thread-safe logical boolean value which can be `set` and `unset`.
 ///