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: I14533676d5774ee0d0ee5054ba77d7dac32cff43
diff --git a/.gitignore b/.gitignore
index 61409f4..22fb5c4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -69,3 +69,14 @@
 tests/testsuite
 tests/testsuite.log
 tests/package.m4
+
+# manuals
+doc/manuals/*.html
+doc/manuals/*.svg
+doc/manuals/*.pdf
+doc/manuals/*__*.png
+doc/manuals/*.check
+doc/manuals/generated/
+doc/manuals/osmomsc-usermanual.xml
+doc/manuals/common
+doc/manuals/build