Use hack to avoid having to put #undef HAVE_ST_MTIM in acconfig.h manually.

m4/st_mtim.m4

@@ -1,4 +1,4 @@
-#serial 1
+#serial 2
 
 dnl From Paul Eggert.
 
@@ -8,7 +8,22 @@ AC_DEFUN(AC_STRUCT_ST_MTIM,
  [AC_CACHE_CHECK([for st_mtim in struct stat], ac_cv_struct_st_mtim,
    [AC_TRY_COMPILE([#include <sys/types.h>
 #include <sys/stat.h>], [struct stat s; s.st_mtim;],
-     ac_cv_struct_st_mtim=yes, ac_cv_struct_st_mtim=no)])
+     ac_cv_struct_st_mtim=yes,
+     ac_cv_struct_st_mtim=no)])
+
   if test $ac_cv_struct_st_mtim = yes; then
-   AC_DEFINE(HAVE_ST_MTIM)
-  fi])
+    if test x = y; then
+      # This code is deliberately never run via ./configure.
+      # FIXME: this is a hack to make autoheader put the corresponding
+      # HAVE_* undef for this symbol in config.h.in.  This saves me the
+      # trouble of having to add the #undef in acconfig.h manually.
+      AC_CHECK_FUNCS(ST_MTIM)
+    fi
+    # Defining it this way (rather than via AC_DEFINE) short-circuits the
+    # autoheader check -- autoheader doesn't know it's already been taken
+    # care of by the hack above.
+    ac_kludge=HAVE_ST_MTIM
+    AC_DEFINE_UNQUOTED($ac_kludge)
+  fi
+ ]
+)