have been disabled.

* Makefile.am (SUBDIRS): Rewrite using the MAYBE_ idiom.  Recurse
into taf-kit conditionally.
* taf-kit/src/Makefile.am (bin_PROGRAMS): Build all programs
unconditionally.
* build-aux/vcsn-taf-kit.m4 (VCSN_TAF_KIT): Rename as ...
(ENABLE_TAF_KIT): ... for consistency.