libc/glib: pass installation subdir to backend

When building a multilib variant, install in a separate directory, to
avoid clutering the default or any other variant.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

reporter_name="Harold Grovesteen"

reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00052.html"

reporter_comment=""