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: Ic3c5add3c87f0aadb1ffab668ce16be6d0805d33
diff --git a/doc/Makefile.am b/doc/Makefile.am
index 5a23107..15f36b7 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -1,3 +1,4 @@
 SUBDIRS = \
 	examples \
+	manuals \
 	$(NULL)