To allow user to properly configure the location of lxcpath (/var/lxc),
it should be build based on variable 'localstatedir' instead of 'prefix'.

By default, localstatedir=$prefix/var
localstatedir can be set by the user during configure:

./configure --localstatedir=/var
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>