[build] Fixing out of source build

Package config files that are getting generated should use
CMAKE_BINARY_DIR for install commands. If not, out of source build fails
[build] Specifying VST file names in the install command

The GLOB and file hack used previously is not working when packaging
.deb files. Fixing this by specifying full path to the vst files
[learn] Using WAL as the journal mode while learning

Before this commit, learn-from-file was using MEMORY as the journal
mode. This was locking the whole database making read and write blocked
till the learn-from file finishes.

From this commit onwards, we use WAL mode so that learn and
transliteration can continue together.
[varnamc] varnamruby.rb will be loaded from the standard search path

Before this commit, varnamc does a require_relative and expected the
varnamruby.rb to be in the same path as varnamc. But varnamruby.rb is
not an executable but a library and should not be installd at bin.
Debian lintian was giving warning about this.

This commit adds a set of predefined search paths, ., /usr/local/lib and
/usr/lib. The first matched path will be taken into account
[release] v3.1.2

* Fixed a bug which was causing transliteration to fail while learn was
* happening
* Fixed out of source build
* Fixed varnamc export
* Using 30000 as export size
* Installing varnamruby.rb to the correct location (prefix/lib)