Log Message:
Drop guile support because modern guile isn't supported.

siag builds with guile 1.6, but not 1.8.  Because 1.6 will be dropped
from pkgsrc soonish as unreasonably old, disable guile support.  (siag
itself is no longer maintained and thus a candidate for eventual
removal if it causes trouble.)

siag has an included scheme (SIOD), so is still functional without guile.