version supports all versions of LLVM except 3.6.

While here:

	* Make detection of the installed LLVM automatic, using the
	  latest installed or 3.5 if none are present. It is possible
	  to supply a different value by setting LLVM_VERSION.
	  (Somebody ought to port llvm to the DEFAULT_VERSIONS framework.)
	* Do not even download the docs, if the DOCS-option is off.
	* If DOCS-option is on, do not install the *.tm files.
	* Force the tool to look for documentation in ${DOCSDIR}.
	* Add the functional regression-check target to Makefile.

Approved by:	being condemned too often for too long