Fix compilation on systems with empty _POSIX_TIMERS

Systems such as Tru64 ship with broken headers that
have _POSIX_TIMERS defined but empty.

CLA: trivial

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15598)

providers/implementations/rands/seeding/rand_unix.c

@@ -76,7 +76,9 @@ static uint64_t get_timer_bits(void);
  * macro that might be undefined.
  */
 # undef OSSL_POSIX_TIMER_OKAY
-# if defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0
+/* On some systems, _POSIX_TIMERS is defined but empty.
+ * Subtracting by 0 when comparing avoids an error in this case. */
+# if defined(_POSIX_TIMERS) && _POSIX_TIMERS -0 > 0
 #  if defined(__GLIBC__)
 #   if defined(__GLIBC_PREREQ)
 #    if __GLIBC_PREREQ(2, 17)