config: better (i.e. more portable) test for libxml2

Over the weekend I tried to build on MacOS X¹ and ran into the following
issues:
1) The recent change to autogen.sh to test for pkg-config falls down.
2) After removing the pkg-config test in autogen.sh, w/o pkg-config the
   PKG_CHECK_MODULES macro invocation in configure[.ac] falls down. N.B.
   Solaris users run into this too, even through there's a (broken)
   pkg-config package that can be installed.
3) There are other problems in the code related to fuse that are beyond the
   scope of this.

It seems that pkg-config is only a requirement for the definition of the
PKG_CHECK_MODULES macro used to detect libxml2. Since this seems to be
inherently unportable — at least to MacOS X and Solaris — I'd like to:
A) Change the use of the PKG_CHECK_MODULES macro to the more portable
   AM_PATH_XML2 macro provided by the libxml2 package in
   /usr/.../share/aclocal/libxml.m4
2) Revisit the decision to add the check for pkg-config in autogen.sh in
   BZ 921817.

For now this is just an rfc. If people are agreeable I'll reenter this
change against BZ 921817.

¹Mountain Lion 10.8.3, XCode 4.6.1

Change-Id: I237b1ed8919088345b8fd943423b2a6ad289981b
BUG: 921817
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
Reviewed-on: http://review.gluster.org/4720
Reviewed-by: Justin Clift <jclift@redhat.com>
Tested-by: Justin Clift <jclift@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>