Makefile.am: omit $(top_builddir) from DEPS and LDADDS

This changeset is an attempt to support BSD make.

BSD make is less liberal than GNU make in matching targets, and so it tries to resolve the dependency ./libxml2.la and fails to match the target libxml2.la.

The problem is illustrated by running make on FreeBSD:

vagrant@freebsd:~/libxml2 % make
make  all-recursive
Making all in include
Making all in libxml
Making all in private
Making all in .
  CC       xmllint-xmllint.o
In file included from xmllint.c:24:
/usr/include/sys/timeb.h:43:2: warning: "this file includes <sys/timeb.h> which is deprecated" [-W#warnings]
#warning "this file includes <sys/timeb.h> which is deprecated"
 ^
1 warning generated.
make[2]: don't know how to make ./libxml2.la. Stop

make[2]: stopped in /home/vagrant/libxml2
*** Error code 1

The problem was reported and diagnosed in https://github.com/sparklemotion/nokogiri/issues/1941#issuecomment-561368847, and for the last few years the Nokogiri project has been applying a slightly different patch from https://github.com/sparklemotion/nokogiri/pull/1953

This merge request is an attempt to remove that downstream patch.