bpo-34121: Fix detection of C11 atomic support on clang. (GH-8288)

diff --git a/aclocal.m4 b/aclocal.m4
index 8ed232f..6a24d8e 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -12,9 +12,9 @@
 # PARTICULAR PURPOSE.
 
 m4_ifndef([AC_CONFIG_MACRO_DIRS], [m4_defun([_AM_CONFIG_MACRO_DIRS], [])m4_defun([AC_CONFIG_MACRO_DIRS], [_AM_CONFIG_MACRO_DIRS($@)])])
-# pkg.m4 - Macros to locate and utilise pkg-config.   -*- Autoconf -*-
-# serial 12 (pkg-config-0.29.2)
-
+dnl pkg.m4 - Macros to locate and utilise pkg-config.   -*- Autoconf -*-
+dnl serial 11 (pkg-config-0.29.1)
+dnl
 dnl Copyright © 2004 Scott James Remnant <scott@netsplit.com>.
 dnl Copyright © 2012-2015 Dan Nicholson <dbn.lists@gmail.com>
 dnl
@@ -55,7 +55,7 @@
 dnl See the "Since" comment for each macro you use to see what version
 dnl of the macros you require.
 m4_defun([PKG_PREREQ],
-[m4_define([PKG_MACROS_VERSION], [0.29.2])
+[m4_define([PKG_MACROS_VERSION], [0.29.1])
 m4_if(m4_version_compare(PKG_MACROS_VERSION, [$1]), -1,
     [m4_fatal([pkg.m4 version $1 or higher is required but ]PKG_MACROS_VERSION[ found])])
 ])dnl PKG_PREREQ
@@ -156,7 +156,7 @@
 AC_ARG_VAR([$1][_LIBS], [linker flags for $1, overriding pkg-config])dnl
 
 pkg_failed=no
-AC_MSG_CHECKING([for $2])
+AC_MSG_CHECKING([for $1])
 
 _PKG_CONFIG([$1][_CFLAGS], [cflags], [$2])
 _PKG_CONFIG([$1][_LIBS], [libs], [$2])
@@ -166,11 +166,11 @@
 See the pkg-config man page for more details.])
 
 if test $pkg_failed = yes; then
-        AC_MSG_RESULT([no])
+   	AC_MSG_RESULT([no])
         _PKG_SHORT_ERRORS_SUPPORTED
         if test $_pkg_short_errors_supported = yes; then
 	        $1[]_PKG_ERRORS=`$PKG_CONFIG --short-errors --print-errors --cflags --libs "$2" 2>&1`
-        else
+        else 
 	        $1[]_PKG_ERRORS=`$PKG_CONFIG --print-errors --cflags --libs "$2" 2>&1`
         fi
 	# Put the nasty error message in config.log where it belongs
@@ -187,7 +187,7 @@
 _PKG_TEXT])[]dnl
         ])
 elif test $pkg_failed = untried; then
-        AC_MSG_RESULT([no])
+     	AC_MSG_RESULT([no])
 	m4_default([$4], [AC_MSG_FAILURE(
 [The pkg-config script could not be found or is too old.  Make sure it
 is in your PATH or set the PKG_CONFIG environment variable to the full