random is not multithread-safe in Cygwin

[Bruno Haible]

The function 'random' is, unlike 'rand', not marked as not MT-safe in POSIX
[1][2]. Thus it must be multithread-safe [3]:
  "Each function defined in the System Interfaces volume of POSIX.1-2017
   is thread-safe unless explicitly stated otherwise."

And indeed glibc, musl libc, AIX, Android, and even NetBSD implement it in a
multithread-safe way.

On Cygwin 2.9.0 and 3.4.6, it is not multithread-safe.

How to reproduce:
1. Compile the attached program.
   $ x86_64-pc-cygwin-gcc foo.c
2. Run it.
   $ ./a.exe
Expected: No output.
Actual: Output such as
  Expected value #367 not found in multithreaded results.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/initstate.html
[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/rand.html
[3] https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_407
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