Use GNUInstallDirs variables for install destination

Since we depend on CMake 2.8.7 we can do it (it's included in CMake
since 2.8.5)

The main effect is that on Debian/Ubuntu, libraries will be installed
in lib/${arch}/ instead of in lib, enabling installation for
multi-architecture (see http://wiki.debian.org/Multiarch)

Also it makes no longer necessary to remember to use lib${LIB_SUFFIX}
to install in the correct lib dir since it is handled automatically.