build: Make Guile-Git a hard requirement.

* configure.ac: Error out when (git) is missing.
* doc/guix.texi (Requirements): Mention Guile-Git.
* Makefile.am (MODULES): Add guix/git.scm unconditionally.

Makefile.am

@@ -66,6 +66,7 @@ MODULES =					\
   guix/gnu-maintenance.scm			\
   guix/upstream.scm				\
   guix/licenses.scm				\
+  guix/git.scm					\
   guix/graph.scm				\
   guix/cache.scm				\
   guix/cve.scm					\
@@ -207,13 +208,6 @@ MODULES +=					\
 
 endif HAVE_GUILE_SSH
 
-if HAVE_GUILE_GIT
-
-MODULES +=					\
-  guix/git.scm
-
-endif HAVE_GUILE_GIT
-
 if BUILD_DAEMON_OFFLOAD
 
 MODULES +=					\

configure.ac

@@ -100,14 +100,16 @@ if test "x$have_gnutls" != "xyes"; then
   AC_MSG_ERROR([The Guile bindings of GnuTLS are missing; please install them.])
 fi
 
+dnl Check for Guile-Git.
+GUILE_MODULE_AVAILABLE([have_guile_git], [(git)])
+if test "x$have_guile_git" != "xyes"; then
+  AC_MSG_ERROR([Guile-Git is missing; please install it.])
+fi
+
 dnl Guile-JSON is used in various places.
 GUILE_MODULE_AVAILABLE([have_guile_json], [(json)])
 AM_CONDITIONAL([HAVE_GUILE_JSON], [test "x$have_guile_json" = "xyes"])
 
-dnl Check for Guile-Git.
-GUILE_MODULE_AVAILABLE([have_guile_git], [(git)])
-AM_CONDITIONAL([HAVE_GUILE_GIT], [test "x$have_guile_git" = "xyes"])
-
 dnl Make sure we have a full-fledged Guile.
 GUIX_ASSERT_GUILE_FEATURES([regex posix socket net-db threads])
 

doc/guix.texi

@@ -580,6 +580,10 @@ later, including 2.2.x;
 @uref{http://gnutls.org/, GnuTLS}, specifically its Guile bindings
 (@pxref{Guile Preparations, how to install the GnuTLS bindings for
 Guile,, gnutls-guile, GnuTLS-Guile});
+@item
+@c FIXME: Specify a version number once a release has been made.
+@uref{https://gitlab.com/guile-git/guile-git, Guile-Git}, from August
+2017 or later;
 @item @url{http://www.gnu.org/software/make/, GNU Make}.
 @end itemize