configure: check that errno is threadsafe

Without `-pthread` specified to the compiler, errno is not threadsafe on
Solaris (as _REENTRANT is undefined, and _POSIX_C_SOURCE may not be set
until after the inclusion of <errno.h>).

Work on #3340.

configure.ac

@@ -536,6 +536,34 @@ 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(