[libxml++] docs: manual: Fix srcdir!=buildir build.

[Murray Cumming <murrayc src gnome org>]

commit e31b3f61a5dd0cb9e0d51c03511faef1409c48df
Author: Murray Cumming <murrayc murrayc com>
Date:   Sun Oct 22 22:03:50 2017 +0200

    docs: manual: Fix srcdir!=buildir build.

 docs/Makefile.am |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/docs/Makefile.am b/docs/Makefile.am
index edd69f4..8011c32 100644
--- a/docs/Makefile.am
+++ b/docs/Makefile.am
@@ -21,8 +21,8 @@ doc_input = $(addprefix $(top_srcdir)/libxml++/,$(h_sources_public))
 # Sets dist_noinst_DATA, DISTCLEANFILES and MAINTAINERCLEANFILES
 include $(top_srcdir)/build/doc-reference.am
 
-manual/libxml++.xml: manual/libxml++_without_code.xml manual/insert_example_code.pl
-       $(AM_V_GEN)$(PERL) -- manual/insert_example_code.pl ../examples $< >$@
+manual/libxml++.xml: ${top_srcdir}/docs/manual/libxml++_without_code.xml 
${top_srcdir}/docs/manual/insert_example_code.pl
+       $(AM_V_GEN)$(PERL) -- ${top_srcdir}/docs/manual/insert_example_code.pl ../examples $< >$@
 
 manualdir = $(docdir)/manual
 manual_htmldir = $(manualdir)/html