[svn-r6759] fix bug that made OPA mandatory even for ch3:sock

Pass --with-atomic-primitives=auto_allow_emulation to the OPA configure
unless the user specifies a particular value.

This is a regression introduced in r6223, between 1.2.1p1 and 1.3a1.

Reviewed by balaji@.

configure.in

@@ -1005,7 +1005,19 @@ if test -e "$opadir" ; then
     EXTERNAL_SRC_DIRS="$EXTERNAL_SRC_DIRS src/openpa"
     CPPFLAGS="$CPPFLAGS -I${use_top_srcdir}/src/openpa/src -I${master_top_builddir}/src/openpa/src"
 
-    PAC_CONFIG_SUBDIR(src/openpa,,AC_ERROR(OpenPA configure failed))
+    # OPA defaults to "auto", but in MPICH2 we want "auto_allow_emulation" to
+    # easily permit using channels like ch3:sock that don't care about atomics
+    AC_ARG_WITH([atomic-primitives],
+                [AS_HELP_STRING([--with-atomic-primitives],
+                                [Force OPA to use a specific atomic primitives
+                                 implementation.  See the src/openpa directory
+                                 for more info.])],
+                [],[with_atomic_primitives=not_specified])
+    opa_subdir_args=""
+    if test "$with_atomic_primitives" = "not_specified" ; then
+        opa_subdir_args="--with-atomic-primitives=auto_allow_emulation"
+    fi
+    PAC_CONFIG_SUBDIR_ARGS([src/openpa],[$opa_subdir_args],[],[AC_ERROR([OpenPA configure failed])])
     PAC_PREPEND_FLAG([-lopa],[WRAPPER_LIBS])
 
     other_install_dirs="$other_install_dirs src/openpa"