doc: make doc-only -> fallback to user binary

After the #3888 it was not possible to "make doc-only"
in some situations. This now fallsback to any installed
node version and throws "node not found" in error case.

Ref: #3888

PR-URL: #6906
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Makefile

@@ -272,11 +272,15 @@ out/doc/api/assets/%: doc/api_assets/% out/doc/api/assets/
 out/doc/%: doc/%
 	cp -r $< $@
 
+# check if ./node is actually set, else use user pre-installed binary
+gen-json = tools/doc/generate.js --format=json $< > $@
 out/doc/api/%.json: doc/api/%.md
-	$(NODE) tools/doc/generate.js --format=json $< > $@
+	[ -x $(NODE) ] && $(NODE) $(gen-json) || node $(gen-json)
 
+# check if ./node is actually set, else use user pre-installed binary
+gen-html = tools/doc/generate.js --node-version=$(FULLVERSION) --format=html --template=doc/template.html $< > $@
 out/doc/api/%.html: doc/api/%.md
-	$(NODE) tools/doc/generate.js --node-version=$(FULLVERSION) --format=html --template=doc/template.html $< > $@
+	[ -x $(NODE) ] && $(NODE) $(gen-html) || node $(gen-html)
 
 docopen: out/doc/api/all.html
 	-google-chrome out/doc/api/all.html