Fix make dist in the latet doc build.

TEST=Manually

Review URL: https://codereview.appspot.com/6615051

docs/reference/ibus/Makefile.am

@@ -17,8 +17,8 @@ DOC_MAIN_SGML_FILE=$(DOC_MODULE)-docs.sgml
 # The directory containing the source code. Relative to $(srcdir).
 # gtk-doc will search all .c & .h files beneath here for inline comments
 # documenting the functions and macros.
-# e.g. DOC_SOURCE_DIR=../../../gtk
-DOC_SOURCE_DIR=../../../src
+# e.g. DOC_SOURCE_DIR=$(top_srcdir)/gtk
+DOC_SOURCE_DIR=$(top_srcdir)/src
 
 # Extra options to pass to gtkdoc-scangobj. Not normally needed.
 SCANGOBJ_OPTIONS=