This improves compilation speed since scripts are not compiled twice.
When building with ‘make -j4‘ from a clean repository on an Intel
i5-2540M, the time of compilation gets from:

real	5m7.220s
user	18m49.788s
sys	0m12.964s

down to:

real	2m20.820s
user	7m25.992s
sys	0m7.464s

* configure.ac (BUILD_FROM_GIT): New Automake conditional.
* doc.am (SUBCOMMANDS): Delete variable.
(dist_man1_MANS): List all subcommands man pages.
(doc/guix.1): Build only if BUILD_FROM_GIT.  Depend on 'scripts/guix'
instead of all subcommands.
[BUILD_DAEMON] (doc/guix-daemon): Likewise.  Replace the
'nix/nix-daemon/guix-daemon.cc' prerequisite with 'guix-daemon'.
[BUILD_FROM_GIT] (gen_man): New variable.
[BUILD_FROM_GIT] (doc/guix-%.1): New target.
(CLEANFILES) [BUILD_FROM_GIT]: Add $(dist_man1_MANS).
---
 configure.ac |  4 +++
 doc.am       | 90 +++++++++++++++++++++++++++++-------------------------------
 2 files changed, 47 insertions(+), 47 deletions(-)