minor, redefine MYSCM_SYNTAX macro from scratch

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'bld/guile-2.0' into next

* bld/guile-2.0:
  minor, redefine MYSCM_SYNTAX macro from scratch
  minor, use string literals for SCM_SYMBOL to aid building with guile-2.0
  chore, adapt SCM_SYNTAX macro to support both guile-2.0 and guile-1.8