Revert "configure: check that errno is threadsafe"

This reverts commit ee15f1b987.

The test relies on undefined behaviour (checking for errno in the
absence of an error condition) and was broken on OpenBSD.

Closes #4184.

configure.ac

@@ -579,34 +579,6 @@ else
   AC_MSG_RESULT(no)
 fi
 
-# Check that threads actually work on Solaris
-AC_MSG_CHECKING([for threadsafe errno])
-AC_RUN_IFELSE(
-  [AC_LANG_PROGRAM([
-    #include <errno.h>
-    #include <pthread.h>
-    #include <signal.h>
-
-    void *thread1_func(void *p_arg)
-    {
-            errno = 1;
-            return 0;
-    }
-    ],[
-    errno = 0;
-    pthread_t t1;
-    pthread_create(&t1, NULL, thread1_func, NULL);
-    pthread_join(t1, NULL);
-    return errno;
-  ])],
-  [AC_MSG_RESULT(yes)],
-  [
-    AC_MSG_RESULT(no)
-    AC_MSG_FAILURE([errno is not threadsafe - check your compiler settings])
-  ],
-  [AC_MSG_RESULT(crosscompiling, skipped)]
-)
-
 pcre2_min_version=10.21
 EXTRA_PCRE2=
 AC_ARG_WITH(