build: Fix build with old automake

Ancient automake (such as from RHEL5) does not provide abs_srcdir and
abs_builddir variables which are used by a recent commit of mine
(e562e82f).
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>

src/Makefile.am

@@ -16,6 +16,10 @@
 ## License along with this library.  If not, see
 ## <http://www.gnu.org/licenses/>.
 
+# old automake does not provide abs_{src,build}dir variables
+abs_builddir = $(shell pwd)
+abs_srcdir = $(shell cd $(srcdir) && pwd)
+
 # No libraries with the exception of LIBXML should be listed
 # here. List them against the individual XXX_la_CFLAGS targets
 # that actually use them. Also keep GETTEXT_CPPFLAGS at the end.