diff --git a/ares_setup.h b/ares_setup.h
index 58964371..ce81b1fa 100644
--- a/ares_setup.h
+++ b/ares_setup.h
@@ -48,6 +48,17 @@
 /* system header files in our config files, avoid this at any cost. */
 /* ================================================================ */
 
+/*
+ * AIX 4.3 and newer needs _THREAD_SAFE defined to build
+ * proper reentrant code. Others may also need it.
+ */
+
+#ifdef NEED_THREAD_SAFE
+#  ifndef _THREAD_SAFE
+#    define _THREAD_SAFE
+#  endif
+#endif
+
 /*
  * Tru64 needs _REENTRANT set for a few function prototypes and
  * things to appear in the system header files. Unixware needs it
diff --git a/configure.ac b/configure.ac
index 8f1cddb5..f1f52a5b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -82,6 +82,7 @@ dnl This defines _ALL_SOURCE for AIX
 CARES_CHECK_AIX_ALL_SOURCE
 
 dnl Our configure and build reentrant settings
+CARES_CONFIGURE_THREAD_SAFE
 CARES_CONFIGURE_REENTRANT
 
 dnl check for how to do large files
@@ -384,21 +385,6 @@ AC_HELP_STRING([--enable-libgcc],[use libgcc when linking]),
 dnl Default is to try the thread-safe versions of a few functions
 OPT_THREAD=on
 
-dnl detect AIX 4.3 or later
-AC_MSG_CHECKING([AIX 4.3 or later])
-AC_PREPROC_IFELSE([
-#if defined(_AIX) && defined(_AIX43)
-printf("just fine");
-#else
-#error "this is not AIX 4.3 or later"
-#endif
-],
- [ AC_MSG_RESULT([yes])
-   RECENTAIX=yes
-   OPT_THREAD=off ],
- [ AC_MSG_RESULT([no]) ]
-)
-
 AC_ARG_ENABLE(thread,dnl
 AC_HELP_STRING([--disable-thread],[don't look for thread-safe functions])
 AC_HELP_STRING([--enable-thread],[look for thread-safe functions]),
@@ -427,12 +413,6 @@ dnl Let's hope this split URL remains working:
 dnl http://publibn.boulder.ibm.com/doc_link/en_US/a_doc_lib/aixprggd/ \
 dnl genprogc/thread_quick_ref.htm
 
-if test "x$RECENTAIX" = "xyes"; then
-
-  AC_DEFINE(_THREAD_SAFE, 1, [define this if you need it to compile thread-safe code])
-
-fi
-
 
 dnl **********************************************************************
 dnl Back to "normal" configuring
diff --git a/m4/cares-reentrant.m4 b/m4/cares-reentrant.m4
index 5793b779..2c24e557 100644
--- a/m4/cares-reentrant.m4
+++ b/m4/cares-reentrant.m4
@@ -422,8 +422,8 @@ dnl must be unconditionally done for this platform.
 dnl Internal macro for CARES_CONFIGURE_REENTRANT.
 
 AC_DEFUN([CARES_CHECK_NEED_REENTRANT_SYSTEM], [
-  case $host in
-    *-*-solaris*)
+  case $host_os in
+    solaris*)
       tmp_need_reentrant="yes"
       ;;
     *)
@@ -433,6 +433,29 @@ AC_DEFUN([CARES_CHECK_NEED_REENTRANT_SYSTEM], [
 ])
 
 
+dnl CARES_CHECK_NEED_THREAD_SAFE_SYSTEM
+dnl -------------------------------------------------
+dnl Checks if the preprocessor _THREAD_SAFE definition
+dnl must be unconditionally done for this platform.
+dnl Internal macro for CARES_CONFIGURE_THREAD_SAFE.
+
+AC_DEFUN([CARES_CHECK_NEED_THREAD_SAFE_SYSTEM], [
+  case $host_os in
+    aix[[123]].* | aix4.[[012]].*)
+      dnl aix 4.2 and older
+      tmp_need_thread_safe="no"
+      ;;
+    aix*)
+      dnl AIX 4.3 and newer
+      tmp_need_thread_safe="yes"
+      ;;
+    *)
+      tmp_need_thread_safe="no"
+      ;;
+  esac
+])
+
+
 dnl CARES_CONFIGURE_FROM_NOW_ON_WITH_REENTRANT
 dnl -------------------------------------------------
 dnl This macro ensures that configuration tests done
@@ -453,6 +476,26 @@ _EOF
 ])
 
 
+dnl CARES_CONFIGURE_FROM_NOW_ON_WITH_THREAD_SAFE
+dnl -------------------------------------------------
+dnl This macro ensures that configuration tests done
+dnl after this will execute with preprocessor symbol
+dnl _THREAD_SAFE defined. This macro also ensures that
+dnl the generated config file defines NEED_THREAD_SAFE
+dnl and that in turn setup.h will define _THREAD_SAFE.
+dnl Internal macro for CARES_CONFIGURE_THREAD_SAFE.
+
+AC_DEFUN([CARES_CONFIGURE_FROM_NOW_ON_WITH_THREAD_SAFE], [
+AC_DEFINE(NEED_THREAD_SAFE, 1,
+  [Define to 1 if _THREAD_SAFE preprocessor symbol must be defined.])
+cat >>confdefs.h <<_EOF
+#ifndef _THREAD_SAFE
+#  define _THREAD_SAFE
+#endif
+_EOF
+])
+
+
 dnl CARES_CONFIGURE_REENTRANT
 dnl -------------------------------------------------
 dnl This first checks if the preprocessor _REENTRANT
@@ -513,3 +556,56 @@ AC_DEFUN([CARES_CONFIGURE_REENTRANT], [
   #
 ])
 
+
+dnl CARES_CONFIGURE_THREAD_SAFE
+dnl -------------------------------------------------
+dnl This first checks if the preprocessor _THREAD_SAFE
+dnl symbol is already defined. If it isn't currently
+dnl defined a set of checks are performed to verify
+dnl if its definition is required. Finally, if
+dnl _THREAD_SAFE is already defined or needed it takes
+dnl care of making adjustments necessary to ensure
+dnl that it is defined equally for further configure
+dnl tests and generated config file.
+
+AC_DEFUN([CARES_CONFIGURE_THREAD_SAFE], [
+  AC_PREREQ([2.50])dnl
+  #
+  AC_MSG_CHECKING([if _THREAD_SAFE is already defined])
+  AC_COMPILE_IFELSE([
+    AC_LANG_PROGRAM([[
+    ]],[[
+#ifdef _THREAD_SAFE
+      int dummy=1;
+#else
+      force compilation error
+#endif
+    ]])
+  ],[
+    AC_MSG_RESULT([yes])
+    tmp_thread_safe_initially_defined="yes"
+  ],[
+    AC_MSG_RESULT([no])
+    tmp_thread_safe_initially_defined="no"
+  ])
+  #
+  if test "$tmp_thread_safe_initially_defined" = "no"; then
+    AC_MSG_CHECKING([if _THREAD_SAFE is actually needed])
+    CARES_CHECK_NEED_THREAD_SAFE_SYSTEM
+    if test "$tmp_need_thread_safe" = "yes"; then
+      AC_MSG_RESULT([yes])
+    else
+      AC_MSG_RESULT([no])
+    fi
+  fi
+  #
+  AC_MSG_CHECKING([if _THREAD_SAFE is onwards defined])
+  if test "$tmp_thread_safe_initially_defined" = "yes" ||
+    test "$tmp_need_thread_safe" = "yes"; then
+    CARES_CONFIGURE_FROM_NOW_ON_WITH_THREAD_SAFE
+    AC_MSG_RESULT([yes])
+  else
+    AC_MSG_RESULT([no])
+  fi
+  #
+])