meson: introduce IN_LIBLXC preprocessor macro

meson: introduce IN_LIBLXC preprocessor macro

The purpose of it is to tell us if we are compiling
liblxc or lxc test/tool/command.

This thing is needed to exclude unnecessary functions
from being compiled-in in the resulting executables
like lxc-start, lxc-attach, etc.

The problem is that lxc tools (lxc-start, lxc-stop, etc)
depend not only on the liblxc as a shared library, but also
require some non-exported symbols or helpers from liblxc
internals. So, we have to link these executables with some liblxc
object files directly which results in the dependency hell,
because linking one .c file from liblxc may end up having to
link with another one (what contains some dependency) and so on.
By using IN_LIBLXC in the liblxc internals we can selectively
omit some functions from being compiled in such cases.

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>