fix minor configure error for debug builds redux

1. Backed out change from commit 8178acfe.
2. AX_DEBUG calling AX_ADD_AM_MACRO incorrectly. Syntax should be
   AX_ADD_AM_MACRO( VAR += value ), not AX_ADD_AM_MACRO( value, VAR ).

m4/ax_append_to_file.m4

@@ -23,5 +23,5 @@
 
 AC_DEFUN([AX_APPEND_TO_FILE],[
 AC_REQUIRE([AX_FILE_ESCAPES])
-printf -- "$2\n" >> "$1"
+printf "$2\n" >> "$1"
 ])

m4/ax_debug.m4

@@ -52,8 +52,8 @@ AC_DEFUN([AX_DEBUG],
         [Add debug code/turns off optimizations (yes|no) @<:@default=no@:>@])],
       [ax_enable_debug=yes
       AC_DEFINE([DEBUG],[1],[Define to 1 to enable debugging code.])
-      AX_ADD_AM_MACRO([--debug],[AM_YFLAGS])
-      AX_ADD_AM_MACRO([-D_GLIBCXX_DEBUG],[AM_CPPFLAGS])],
+      AX_ADD_AM_MACRO([AM_YFLAGS += --debug])
+      AX_ADD_AM_MACRO([AM_CPPFLAGS += -D_GLIBCXX_DEBUG])],
       [ax_enable_debug=no
       AC_SUBST([MCHECK])
       AC_DEFINE([DEBUG],[0],[Define to 1 to enable debugging code.])])