How to debug configure scripts with 2.53?

[Lars Hecking]

 I have a configure(.in) that breaks all over the place with autoconf 2.53.
 How can I debug this? I used to just

$ sh -x ./configure >log 2>&1

 and then take a look at log, but this doesn't work anymore. config.log
 is no help either.

 One thing I'm trying to find out is why the PATH arg is ignored in

AC_PATH_PROG([file],[file],,[/usr/local/bin:/bin:/usr/bin:$PATH])

 (The file command exists in both /usr/local/bin and /bin, but only
  the latter is found. The generated configure code looks ok.)