Put error global variables into thread-local storage

Global variable `error_buffer` is used to store a string that is
returned to callers, so there is a race condition if dynamic linking is
invoked from 2 OCaml domains in parallel
Since the error message must be returned, a mutex cannot be used to
prevent the race condition
GNU libc uses that same solution: keep the last error in thread-local
storage; so support for calling dlerror from a different thread than the
one calling dlopen is not to be expected

Co-authored-by: David Allsopp <david.allsopp@metastack.com>

flexdll.c

@@ -47,8 +47,11 @@ typedef struct dlunit {
 } dlunit;
 typedef void *resolver(void*, const char*);
 
-static int error = 0;
-static char error_buffer[256];
+#ifdef _MSC_VER
+#define __thread __declspec(thread)
+#endif
+static __thread int error = 0;
+static __thread char error_buffer[256];
 
 /* Emulate a low-level dlopen-like interface */