build: Add 'HELP2MAN' silent rule.

* Makefile.am (AM_V_HELP2MAN, AM_V_HELP2MAN_, AM_V_HELP2MAN_0): New
variables.
* doc.am (subcommand-manual-target, doc/guix.1, doc/guix-daemon.1): Use
$(AM_V_HELP2MAN).

Makefile.am

@@ -488,3 +488,7 @@ AM_V_DOT_0 = @echo "  DOT     " $@;
 AM_V_EMACS = $(AM_V_EMACS_$(V))
 AM_V_EMACS_ = $(AM_V_EMACS_$(AM_DEFAULT_VERBOSITY))
 AM_V_EMACS_0 = @echo "  EMACS   " $@;
+
+AM_V_HELP2MAN = $(AM_V_HELP2MAN_$(V))
+AM_V_HELP2MAN_ = $(AM_V_HELP2MAN_$(AM_DEFAULT_VERBOSITY))
+AM_V_HELP2MAN_0 = @echo "  HELP2MAN" $@;

doc.am

@@ -95,19 +95,19 @@ dvi-local: ps-local
 # Manual pages.
 
 doc/guix.1: $(SUBCOMMANDS:%=guix/scripts/%.scm)
-	-LANGUAGE= $(top_builddir)/pre-inst-env	\
+	-$(AM_V_HELP2MAN)LANGUAGE= $(top_builddir)/pre-inst-env		\
 	  $(HELP2MAN) --output="$@" guix
 
 # Note: Do not depend on 'guix-daemon' since that would trigger a rebuild even
 # for people building from a tarball.
 doc/guix-daemon.1: nix/nix-daemon/guix-daemon.cc
-	-LANGUAGE= $(top_builddir)/pre-inst-env	\
+	-$(AM_V_HELP2MAN)LANGUAGE= $(top_builddir)/pre-inst-env		\
 	  $(HELP2MAN) --output="$@" guix-daemon
 
 define subcommand-manual-target
 
 doc/guix-$(1).1: guix/scripts/$(1).scm
-	-LANGUAGE= $(top_builddir)/pre-inst-env		\
+	-$$(AM_V_HELP2MAN)LANGUAGE= $(top_builddir)/pre-inst-env	\
 	  $(HELP2MAN) --output="$$@" "guix $(1)"
 
 endef