export.pike: Fix fix_configure

This was broken in multiple ways:

* Did not consider timestamp on aclocal.m4
* Did not consider post_modules
* Did not check configure scripts directly under modules or post_modules
* When fixing modules, it looked for the configure scripts in the build
  directory rather than the source directory
* Did not use the run_autoconfig wrapper

This has caused xenofarm to export snapshots with out of date
configure scripts.