build: add libraries to LIBADD instead of LDFLAGS

For a number of linker flags the order of the object files and the libs
to link against matter (for example -Wl,--as-needed). Make sure that
libraries are added via the LIBADD variable instead of LDFLAGS and
therefore always come after the object files.

Change-Id: I59d114752a0c7664b8678a72082ba5e445497fe5
Signed-off-by: Tiziano Müller <tiziano.mueller@stepping-stone.ch>
Reviewed-on: http://review.gluster.org/8331
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Prashanth Pai <ppai@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>