Fix make distcheck by looking for guile-config in $(srcdir)

make distcheck is currently failing for me with:

In file included from /usr/local/include/libguile/threads.h:35,
                 from /usr/local/include/libguile/async.h:28,
                 from /usr/local/include/libguile.h:37,
                 from ../../examples/box/box.c:22:
/usr/local/include/libguile/pthread-threads.h:33:31: error: libguile/boehm-gc.h: No such file or directory
In file included from /usr/local/include/libguile.h:93,
                 from ../../examples/box/box.c:22:

- which is because it's picking up a load of installed headers,
instead of those in the _inst tree

- which is because there is no output from the preceding guile-config
invocation:

PATH=/home/neil/SW/Guile/ovnight/guile-master-20090924/_inst/bin:$PATH PKG_CONFIG_PATH=/home/neil/SW/Guile/ovnight/guile-master-20090924/_inst/lib/pkgconfig /home/neil/SW/Guile/ovnight/guile-master-20090924/_inst/bin/guile-config compile

- which is because .../_inst/bin/guile-config is empty

- which is because the rule in meta/Makefile.am that generates
guile-config is missing $(srcdir), and so doesn't find
guile-config.in.

* meta/Makefile.am (guile-config): Look for guile-config.in in
  $(srcdir).

meta/Makefile.am

@@ -34,9 +34,9 @@ pkgconfig_DATA = guile-2.0.pc
 aclocaldir = $(datadir)/aclocal
 aclocal_DATA = guile.m4
 
-guile-config: guile-config.in
+guile-config: $(srcdir)/guile-config.in
 	guile="@bindir@/`echo guile | $(SED) -e '$(program_transform_name)'`" ;	\
-	cat guile-config.in							\
+	cat $(srcdir)/guile-config.in							\
 	| $(SED) -e "s,@pkgconfigdir@,$(pkgconfigdir),g ;			\
 	             s,@""PKG_CONFIG@,$(PKG_CONFIG),g ;				\
 	             s,@installed_guile@,$$guile,g"				\