[librsvg] Fix the source path in doc/Makefile.am

[Federico Mena Quintero <federico src gnome org>]

commit c6540063264b2a1012486327c55eaeb9bdf4a220
Author: Federico Mena Quintero <federico gnome org>
Date:   Fri Dec 8 15:52:08 2017 -0600

    Fix the source path in doc/Makefile.am

 doc/Makefile.am |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/doc/Makefile.am b/doc/Makefile.am
index 1aca98b..4620dcf 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -20,7 +20,7 @@ DOC_MAIN_SGML_FILE = $(DOC_MODULE)-docs.xml
 # gtk-doc will search all .c & .h files beneath here for inline comments
 # documenting the functions and macros.
 # e.g. DOC_SOURCE_DIR=../../../gtk
-DOC_SOURCE_DIR = ..
+DOC_SOURCE_DIR = $(abs_top_srcdir)
 
 # Extra options to pass to gtkdoc-scangobj. Not normally needed.
 SCANGOBJ_OPTIONS = --type-init-func="g_type_init ()"