configury: handle missing ldoc binary.

* configure.ac (LDOC): Set with AX_WITH_PROG.
(HAVE_LDOC): New conditional.
* Makefile.am (HTML): Fold into...
(dist_doc_DATA): ...here.
Run ldoc when available, otherwise touch the requried files to
prevent build failure.
Merge pull request #59 from gvvaughan/pull-request/handle-missing-ldoc

configury: handle missing ldoc binary.