build: improve clippy options

* adds a `--with-clippy=...` option to use a prebuilt clippy binary
* limits the autoconf tests done for `--enable-clippy-only`
  (e.g. no libyang)

Fixes: FRRouting#3921
Fixes: FRRouting#4006
Signed-off-by: David Lamparter <equinox@diac24.net>

Makefile.am

@@ -40,6 +40,12 @@ shvar-%:
 var-%:
 	@echo "$($*)" >&$(VARFD)
 
+if ONLY_CLIPPY
+.DEFAULT_GOAL := clippy-only
+endif
+clippy-only: Makefile lib/clippy config.h
+.PHONY: clippy-only
+
 # overwriting these vars breaks cross-compilation.  let's be helpful and warn.
 #
 # note: "#AUTODERP# " will be removed from Makefile by configure.  These are