package/giblib: fix breakage

Fix two things in the giblib package:

1- fix prefix breakage
   - s/STAGING/STAGING_DIR/ (Thomas)
   - prefix is .../usr, not .../usr/lib or .../usr/bin

2- fix the configure.ac/Makefile.am breakage
   - imlib2 sets IMLIB2_(LIBS|CFLAGS), not IMLIB_(LIBS|CFLAGS)
   - autoreconf the package, install missing files
     Note that both configure.ac and configure.in are present in the
     package, so both are fixed, although only configure.ac is used
     (configure.in is the legacy name).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
CC: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>