build manuals moved here from osmo-gsm-manuals.git

Moved to doc/manuals/, with full commit history, in preceding merge commit.
Now incorporate in the build system.

Build with:

$ autoreconf -fi
$ ./configure --enable-manuals
$ make

Shared files from osmo-gsm-manuals.git are found automatically if
- the repository is checked out in ../osmo-gsm-manuals; or
- if it osmo-gsm-manuals was installed with "make install"; or
- OSMO_GSM_MANUALS_DIR is set.

Related: OS#3385
Change-Id: I0477d7c871413bd90b365d3064bac3cba23a0883
diff --git a/doc/manuals/Makefile.am b/doc/manuals/Makefile.am
new file mode 100644
index 0000000..00a08c8
--- /dev/null
+++ b/doc/manuals/Makefile.am
@@ -0,0 +1,16 @@
+EXTRA_DIST = osmosgsn-usermanual.adoc \
+    osmosgsn-usermanual-docinfo.xml \
+    osmosgsn-vty-reference.xml \
+    chapters \
+    vty
+
+if BUILD_MANUALS
+  ASCIIDOC = osmosgsn-usermanual.adoc
+  ASCIIDOC_DEPS = $(srcdir)/chapters/*.adoc
+  include $(OSMO_GSM_MANUALS_DIR)/build/Makefile.asciidoc.inc
+
+  VTY_REFERENCE = osmosgsn-vty-reference.xml
+  include $(OSMO_GSM_MANUALS_DIR)/build/Makefile.vty-reference.inc
+
+  include $(OSMO_GSM_MANUALS_DIR)/build/Makefile.common.inc
+endif