Support `LIB_SUFFIX` in `GNUInstallDirs.cmake`

When installing on multi lib systems it is required to install to either lib, lib32 or lib64.

Currently GNUInstallDirs does some magic trying to figure out if it should be installed to lib, lib64 or some debian specific location.

However it is common to see LIB_SUFFIX as supported as a parameter that allows to configure the lib directory to lib${LIB_SUFFIX}.

It would be good if GNUInstallDirs would support that parameter too.

Possible issues

1. There are are already many Projects manually appending ${LIB_SUFFIX} to their lib. If used in conjunction with GNUInstallDirs these would duplicate the suffix (e.g. lib6464). One possible solution would be to use a different name like CMAKE_LIB_SUFFIX. This if either LIB_SUFFIX or CMAKE_LIB_SUFFIX is set it would work. However it would still duplicate if both are set.