enable tr1 headers if not explicitly disabled (this is the semantics described

the the help string)

[[Imported from SVN: r5653]]

m4/dune_common.m4

@@ -49,7 +49,7 @@ AC_DEFUN([DUNE_COMMON_CHECKS],
 	AS_HELP_STRING([--disable-tr1-headers],
 	[Prevents checking for  tr1 headers like tuple, array, etc. 
 	This might be needed for broken compilers like xlc on Blue Gene]))
-  AS_IF([test "x$enable_tr1_headers" = "xyes"],[
+  AS_IF([test "x$enable_tr1_headers" != "xno"],[
 	  AC_CHECK_HEADERS([type_traits tr1/type_traits array tr1/array tuple tr1/tuple])
   ])