build: AM_PATH_XML2 needs proactive check

AM_PATH_XML2 needs check for its existence using
aclocal macros - to avoid problems like below when
libxml2 development libraries are not installed

--------------------------------------------------

... GlusterFS autogen ...

Generate gf-error-codes.h ...
`gf-error-codes.h' -> `libglusterfs/src/gf-error-codes.h'
Running aclocal...
configure.ac:524: warning: macro `AM_PATH_XML2' not found in library
Running autoheader...
Running libtoolize...
Running autoconf...
configure.ac:524: error: possibly undefined macro: AM_PATH_XML2
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
Running automake...
--------------------------------------------------

Change-Id: Ife463c34c45babc1c4c0ed6e8128b5c43419b9b9
BUG: 947226
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/5962
Reviewed-by: Kaushal M <kaushal@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>