Make _Thread_local work on windows haven't tested tho

inc/errno.h

@@ -5,5 +5,4 @@
 #define EILSEQ 2
 #define ERANGE 3
 
-// TODO: make it thread-local
-extern int errno;
+extern _Thread_local int errno;

src/code/errno.c

@@ -1,4 +1,4 @@
 
 #include <errno.h>
 
-int errno;
+_Thread_local int errno;

src/win/win_threads.c

@@ -8,6 +8,8 @@
 #include <win.h>
 #include <threads.h>
 
+DWORD _tls_index = 0;
+
 void mtx_destroy(mtx_t *mtx) {
     CloseHandle(mtx->semaphore);
 }

test/test_assert.c

@@ -1,7 +1,7 @@
 
 #include <assert.h>
 #include <stddef.h>
-
+#include <errno.h>
 void do_more_stuff(char *ptr) {
     assert(ptr != NULL);
 }
@@ -11,6 +11,7 @@ void do_stuff() {
 }
 
 int main() {
+    errno = 0;
     assert(2+2 == 4);
     do_stuff();
 }