Further fixes for detecting GStreamer version

- Suppress annoying error when building without sound support
- Require Autoconf 2.64
- Use detected grep path
Define Inform 6 version in configure.ac

6.32 is the current version, but since the version name is in the
executable, define it in configure.ac and substitute it in the
Makefile.
Various fixes for 'make distcheck'

- app-test.c used disappeared API
- There were new files added in Chimara that had strings marked for
  translation
- For some reason the --enable-distcheck option wasn't working