build: Build man pages in $(srcdir).

Before that the doc/guix.1 rule was always triggered when doing a VPATH
build from a tarball.

* doc.am (sub_commands_mans, dist_man1_MANS): man pages are now
generated in $(srcdir) like the info manual.
(doc/guix.1, doc/guix-%.1): Prepend $(srcdir) to target name.
[BUILD_DAEMON] (doc/guix-daemon.1): Likewise.

doc.am

@@ -99,25 +99,25 @@ dvi-local: ps-local
 ## ----------- ##
 
 sub_commands_mans =				\
-  doc/guix-archive.1				\
-  doc/guix-build.1				\
-  doc/guix-challenge.1				\
-  doc/guix-download.1				\
-  doc/guix-edit.1				\
-  doc/guix-environment.1			\
-  doc/guix-gc.1					\
-  doc/guix-hash.1				\
-  doc/guix-import.1				\
-  doc/guix-lint.1				\
-  doc/guix-package.1				\
-  doc/guix-publish.1				\
-  doc/guix-pull.1				\
-  doc/guix-refresh.1				\
-  doc/guix-size.1				\
-  doc/guix-system.1
+  $(srcdir)/doc/guix-archive.1			\
+  $(srcdir)/doc/guix-build.1			\
+  $(srcdir)/doc/guix-challenge.1		\
+  $(srcdir)/doc/guix-download.1			\
+  $(srcdir)/doc/guix-edit.1			\
+  $(srcdir)/doc/guix-environment.1		\
+  $(srcdir)/doc/guix-gc.1			\
+  $(srcdir)/doc/guix-hash.1			\
+  $(srcdir)/doc/guix-import.1			\
+  $(srcdir)/doc/guix-lint.1			\
+  $(srcdir)/doc/guix-package.1			\
+  $(srcdir)/doc/guix-publish.1			\
+  $(srcdir)/doc/guix-pull.1			\
+  $(srcdir)/doc/guix-refresh.1			\
+  $(srcdir)/doc/guix-size.1			\
+  $(srcdir)/doc/guix-system.1
 
 dist_man1_MANS =				\
-  doc/guix.1					\
+  $(srcdir)/doc/guix.1				\
   $(sub_commands_mans)
 
 # Man pages are generated using GNU help2man.
@@ -129,14 +129,14 @@ 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 $(sub_commands_mans)
+$(srcdir)/doc/guix.1: scripts/guix.in $(sub_commands_mans)
 	-$(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
 # pages are only generated if the corresponding script source has been
 # changed.
-doc/guix-%.1: guix/scripts/%.scm $(GOBJECTS)
+$(srcdir)/doc/guix-%.1: guix/scripts/%.scm $(GOBJECTS)
 	-@case '$?' in \
 	  *$<*) $(AM_V_P) && set -x || echo "  HELP2MAN $@"; \
 	        $(gen_man) --output="$@" "guix $*";; \
@@ -145,11 +145,11 @@ doc/guix-%.1: guix/scripts/%.scm $(GOBJECTS)
 
 if BUILD_DAEMON
 
-dist_man1_MANS += doc/guix-daemon.1
+dist_man1_MANS += $(srcdir)/doc/guix-daemon.1
 
 # 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
+$(srcdir)/doc/guix-daemon.1: nix/nix-daemon/guix-daemon.cc
 	-$(AM_V_HELP2MAN)$(gen_man) --output="$@" `basename "$@" .1`
 
 endif