Properly define ENABLE_THREAD_SAFETY in conflgure, per suggestion from Peter.

configure

@@ -4881,6 +4881,13 @@ else
 fi
 
 
+if test "$enable_thread_safety" = yes; then
+
+cat >>confdefs.h <<\_ACEOF
+#define ENABLE_THREAD_SAFETY 1
+_ACEOF
+
+fi
 { $as_echo "$as_me:$LINENO: result: $enable_thread_safety" >&5
 $as_echo "$enable_thread_safety" >&6; }
 

configure.in

 dnl Process this file with autoconf to produce a configure script.
-dnl $PostgreSQL: pgsql/configure.in,v 1.615 2009/12/02 14:07:25 momjian Exp $
+dnl $PostgreSQL: pgsql/configure.in,v 1.616 2009/12/11 02:21:21 momjian Exp $
 dnl
 dnl Developers, please strive to achieve this order:
 dnl
@@ -559,6 +559,10 @@ IFS=$ac_save_IFS
 #
 AC_MSG_CHECKING([allow thread-safe client libraries])
 PGAC_ARG_BOOL(enable, thread-safety, yes, [disable thread-safety in client libraries])
+if test "$enable_thread_safety" = yes; then
+  AC_DEFINE([ENABLE_THREAD_SAFETY], 1,
+          [Define to 1 to build client libraries as thread-safe code. (--enable-thread-safety)])
+fi
 AC_MSG_RESULT([$enable_thread_safety])
 AC_SUBST(enable_thread_safety)