Don't barf if the user disables an option that isn't supported.

XXX Should warn instead.

Index: mk/bsd.options.mk
===================================================================
RCS file: /cvsroot/pkgsrc/mk/bsd.options.mk,v
retrieving revision 1.67
diff -p -u -r1.67 bsd.options.mk
--- mk/bsd.options.mk	18 Feb 2008 14:30:40 -0000	1.67
+++ mk/bsd.options.mk	23 Apr 2013 13:29:19 -0000
@@ -284,7 +284,7 @@ _opt_:=		${_popt_}
 _opt_:=		-${_popt_}
 .    endif
 .  endif
-.  if empty(PKG_SUPPORTED_OPTIONS:M${_popt_})
+.  if empty(_opt_:M-*) && empty(PKG_SUPPORTED_OPTIONS:M${_popt_})
 _OPTIONS_UNSUPPORTED:=${_OPTIONS_UNSUPPORTED} ${_opt_}
 .  else
 .    if defined(_PKG_OPTIONS_GROUP_MAP.${_popt_})