build: fix glupy linking warnings

While building glupy on x86_64, the following warnings are thrown:

  Making all in glupy
  Making all in src
    CC     glupy.lo
    CCLD   glupy.la
  /usr/bin/ld: skipping incompatible /usr/lib/libdl.so when searching for -ldl
  /usr/bin/ld: skipping incompatible /usr/lib/libutil.so when searching for -lutil
  /usr/bin/ld: skipping incompatible /usr/lib/libm.so when searching for -lm
  /usr/bin/ld: skipping incompatible /usr/lib/librt.so when searching for -lrt
  /usr/bin/ld: skipping incompatible /usr/lib/libpthread.so when searching for -lpthread
  /usr/bin/ld: skipping incompatible /usr/lib/libc.so when searching for -lc

The libraries to link against are in /usr/lib64 on x86_64 in Fedora,
RHEL and CentOS. Other architectures or distributions may use a
different path. Instead of hardcoding "/lib" in configure.ac, the
variable $libdir should be used so that the path gets adapted to the
distribution and architecture.

Change-Id: Iae5db3ee8a62959ccb7f958d7c95eb77c4d4b1dd
BUG: 1159248
Reported-by: Kiran Patil <kiran@fractalio.com>
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/9033
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>