fix guile-snarf

fix guile-snarf

* libguile/snarf.h: New macro `SCM_SNARF_INIT_PREFIX'. (SCM_SNARF_INIT)
  Use `SCM_SNARF_INIT_PREFIX' instead of including a literal marker. If
  the preprocessor echoes #define directives to its output, this will
  prevent `guile-snarf' from snarfing the `SCM_SNARF_INIT' definition
  itself. Reported by Mike Gran <spk121@yahoo.com>.
* libguile/guile-snarf.in (modern_snarf): Don't output anything for
  lines in which only one of the magic snarfing markers is present.
  Modify the `sed' program for compatibility with POSIX `sed'. The new
  `sed' program is based on a version by Wolfgang Jenkner
  <wjenkner@inode.at>.
* test-suite/standalone/test-guile-snarf: New tests.