[gtksourceview/gnome-3-20] build: fix gtk-doc build when builddir != srcdir

[Sébastien Wilmet <swilmet src gnome org>]

commit 763783c4dfdd2fa059c78cf45f7ad7394e873b8d
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sat May 14 18:28:28 2016 +0200

    build: fix gtk-doc build when builddir != srcdir

 docs/reference/Makefile.am |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/Makefile.am b/docs/reference/Makefile.am
index 9c93b41..884e7e0 100644
--- a/docs/reference/Makefile.am
+++ b/docs/reference/Makefile.am
@@ -20,6 +20,10 @@ DOC_SOURCE_DIR = $(top_srcdir)/gtksourceview
 HFILE_GLOB = $(top_srcdir)/gtksourceview/*.h
 CFILE_GLOB = $(top_srcdir)/gtksourceview/*.c
 
+# Extra header to include when scanning, which are not under DOC_SOURCE_DIR
+# e.g. EXTRA_HFILES = $(top_srcdir}/contrib/extra.h
+EXTRA_HFILES = $(top_builddir)/gtksourceview/gtksourceversion.h
+
 # Header files or dirs to ignore when scanning. Use base file/dir names
 # e.g. IGNORE_HFILES=gtkdebug.h gtkintl.h private_code
 IGNORE_HFILES =                                        \