--- libidn-0.6.14/lib/gl/m4/extensions.m4.aconf262
+++ libidn-0.6.14/lib/gl/m4/extensions.m4
@@ -16,6 +16,7 @@
 # ------------------------
 # Enable extensions on systems that normally disable them,
 # typically due to standards-conformance issues.
+m4_ifndef([AC_USE_SYSTEM_EXTENSIONS], [
 AC_DEFUN([AC_USE_SYSTEM_EXTENSIONS],
 [
   AC_BEFORE([$0], [AC_COMPILE_IFELSE])
@@ -49,6 +50,7 @@ AC_DEFUN([AC_USE_SYSTEM_EXTENSIONS],
   AC_DEFINE([_POSIX_PTHREAD_SEMANTICS])
   AC_DEFINE([_TANDEM_SOURCE])
 ])
+])
 
 # gl_USE_SYSTEM_EXTENSIONS
 # ------------------------