Add some tweaks to the Makefile to make it behave more like a Makefil…

…e ;-)

- use Make variables
- remove empty target file in case of an error (the target file is
  created by the shell redirection before `mandoc` runs)

Makefile

@@ -2,5 +2,5 @@ src=hostmux.mandoc
 tgt=man/hostmux.1
 
 $(tgt): $(src)
-	mkdir -p $(shell dirname $(tgt))
-	mandoc -I os=sh -Tman $(src) > $(tgt)
+	mkdir -p $(shell dirname $@)
+	mandoc -I os=sh -Tman $< > $@ || { rm -f $@ ; exit 2 ; }