Use ${CMAKE_INSTALL_LIBDIR} as lib directory for tng_io library

Directory lib was used as library directory for tng_io library. This
causes problems on Debian wheezy (and future versions), which expect
libraries to be installed under lib/<arch>. GROMACS already uses
${CMAKE_INSTALL_LIBDIR} for libgromacs.so, and this patch makes
libtng_io and libtng_compress install in the same location.

This was also addressed in the TNG repository and this commit
corresponds to commit e8fa7bf3abfc74acf60a048cdfe1f65fdbc0dc2d in
the TNG repository.

This patch also bumps minimum CMake version required for building
tng_io from 2.8 to 2.8.8 due to inclusion of GNUInstallDirs module.

Change-Id: I68b2196ab77e3e18c3a50717327e59b3482f2a29