maint: detect VPATH builds when checking for gnulib update

I accidentally typed 'make' in the srcdir of a VPATH build, and
was surprised to see this:

$ make
/bin/sh: s/^[ +-]//;s/ .*//: No such file or directory
INFO: gnulib update required; running ./autogen.sh first
make: -n: Command not found
./autogen.sh
I am going to run ./configure with no arguments - if you wish
to pass any to it, please specify them on the ./autogen.sh command line.
running bootstrap...
./bootstrap: Bootstrapping from checked-out libvirt sources...
./bootstrap: getting gnulib files...

Oops - we're trying to execute some fairly bogus command names,
and then trying to configure in-tree (which breaks all existing
VPATH builds, since automake refuses to do a VPATH build if it
detects an in-tree configure).  The third line (executing "-n")
is fixed by updating to the latest gnulib; the rest of the problem
is fixed by copying the same filtering in our cfg.mk as what
gnulib just added, so that we avoid any $(shell) invocations which
in turn depend on variables that are only populated by a working
Makefile.  With that in place, we are back to the much nicer:

$ make
There seems to be no Makefile in this directory.
You must run ./configure before running 'make'.
make: *** [abort-due-to-no-makefile] Error 1

Additionally, although harder to see - there was a trailing space in
the message warning us that autogen would run an in-tree configure.

* .gnulib: Update to latest, in part for maint.mk improvements.
* cfg.mk (_update_required): Don't check for update in
unconfigured directory.
* autogen.sh (no_git): Drop trailing space.

Signed-off-by: Eric Blake <eblake@redhat.com>

.gnulib

@@ -1 +1 @@
-Subproject commit e8e0eb6bfb728685ec8d5afd924e41b18e9d928d
+Subproject commit d55899fd2c5794ac85ecb14d5e2f646a89e4b4dd

autogen.sh

@@ -35,7 +35,7 @@ if test -z "$NOCONFIGURE" ; then
     echo "Running ./configure with $EXTRA_ARGS $@"
   else
     if test -z "$*" && test ! -f "$THEDIR/config.status"; then
-        echo "I am going to run ./configure with no arguments - if you wish "
+        echo "I am going to run ./configure with no arguments - if you wish"
         echo "to pass any to it, please specify them on the $0 command line."
     fi
   fi

cfg.mk

@@ -939,6 +939,7 @@ sc_require_locale_h:
 # We don't use this feature of maint.mk.
 prev_version_file = /dev/null
 
+ifneq ($(_gl-Makefile),)
 ifeq (0,$(MAKELEVEL))
   _curr_status = .git-module-status
   # The sed filter accommodates those who check out on a commit from which
@@ -971,6 +972,7 @@ ifeq (0,$(MAKELEVEL))
 maint.mk Makefile: _autogen
   endif
 endif
+endif
 
 # It is necessary to call autogen any time gnulib changes.  Autogen
 # reruns configure, then we regenerate all Makefiles at once.
@@ -980,7 +982,9 @@ _autogen:
 	./config.status
 
 # regenerate HACKING as part of the syntax-check
+ifneq ($(_gl-Makefile),)
 syntax-check: $(top_srcdir)/HACKING bracket-spacing-check
+endif
 
 bracket-spacing-check:
 	$(AM_V_GEN)files=`$(VC_LIST) | grep '\.c$$'`; \