configury: fix hard dependency on luadocs.

* m4/ax_with_prog.m4: New file.
* configure.ac (AX_WITH_PROG): Use it to find a luadocs binary.
* Makefile.am ($(dist_doc_DATA)): Use the substituted binary.
* rockspecs.lua: Remove insertion of luadocs dependency.
Merge pull request #12 from gvvaughan/pull-request/fix-luadocs-hard-dependency

configury: fix hard dependency on luadocs.