vle: allow assign local to POSIX

By default, VLE use the locale according to the environment variables.
In library mode, a "POSIX" or "C" locale can be used. We add a new
constructor to to vle::Init class to permit "POSIX" locale.
vle: add WITH_COVERAGE option in CMake

We need to perform coverage test in VLE. So, we add the gcc `--coverage'
option in CFLAGS, CXXFLAGS and LDFLAGS if the user selects the
WITH_COVERAGE option in CMake.
value: remove boost::serialization depends

Since VLE 1.1 does not (de)serialize value between components, we can
remove this dependency to Boost.
template: assign default path for scripts

Assign a default path scripts Vle*.cmake since CMake complains to not
found these scripts.
utils: split source/binary package

We change the behaviour of packages in VLE. Now, the VLE_HOME directory
stores only the binary version of packages. The source code must be
anywhere on the system.

For example, the source package is in $HOME:
$ cd $HOME
$ vle -P foo create

The binary package goes to the $VLE_HOME/pkgs-1.1 directory:
$ vle -P foo configure build
$ ls $VLE_HOME/pkgs-1.1/foo
Authors.txt  Description.txt  exp          News.txt  plugins
data         doc              License.txt  output    Readme.txt
vle: allow building of VLE with old system

We allow to build VLE (but without Gvle) on old system i.e. with
libarchive 2.8.x and boost and boost asio <= 1.41