build: fix for make distcheck without enable-gtk-doc

Without this patch, the following fails with a rather obscure message
about missing make target.

  ./autogen.sh && make && make distcheck
  ...
  *** No rule to make target `NetworkManager.8', needed by `distdir'. Stop.

Swap the order of the subdirectories 'docs' and 'man' to build
'docs' earlier. This way, `make distcheck` fails in the directory 'docs'
with a better error message:
  *** gtk-doc must be installed and enabled in order to make dist

Also, add 'man/nmcli-examples.5' to the list of files, to determine
whether to use the pre generated doc files.

Signed-off-by: Thomas Haller <thaller@redhat.com>

Makefile.am

@@ -14,8 +14,8 @@ SUBDIRS = \
 	data \
 	test \
 	po \
-	man \
 	docs \
+	man \
 	examples \
 	vapi
 

configure.ac

@@ -688,6 +688,7 @@ install_pregen_manpages=no
 if test "$enable_gtk_doc" != "yes" \
 	-a -f man/NetworkManager.conf.5 \
 	-a -f man/nm-settings.5 \
+	-a -f man/nmcli-examples.5 \
 	-a -f man/NetworkManager.8; then
 	install_pregen_manpages=yes
 fi