Revert "build: Do not remake doc/guix.1."

This reverts commit 97966e6d65.

doc.am

@@ -129,12 +129,8 @@ HELP2MANFLAGS = --source=GNU --info-page=$(PACKAGE_TARNAME)
 
 # Note: Do not depend on 'scripts/guix' since that would trigger a rebuild
 # even for people building from a tarball.
-doc/guix.1: scripts/guix.in $(GOBJECTS)
-	-@case '$?' in \
-	  *$<*) $(AM_V_P) && set -x || echo "  HELP2MAN $@"; \
-	        $(gen_man) --output="$@" "guix";; \
-	  *)    : ;; \
-	esac
+doc/guix.1: $(sub_commands_mans)
+	-$(AM_V_HELP2MAN)$(gen_man) --output="$@" `basename "$@" .1`
 
 # Note: The dependency on $(GOBJECTS) is meant to force these docs to be made
 # only after all Guile modules have been compiled.  The 'case' ensures the man