[PATCH] Fix build with gtk-doc<1.15

[Robert Shade <robert shade gmail com>]

gtk-doc<1.15 does not handle a missing DOC_SOURCE_DIR
---
 docs/api/Makefile.am |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/docs/api/Makefile.am b/docs/api/Makefile.am
index e061ec6..571926f 100644
--- a/docs/api/Makefile.am
+++ b/docs/api/Makefile.am
@@ -55,6 +55,8 @@ DOC_MODULE=NetworkManager
 # The top-level SGML file.
 DOC_MAIN_SGML_FILE=network-manager-docs.xml

+DOC_SOURCE_DIR=$(srcdir)
+
 # Extra options to supply to gtkdoc-mkdb
 MKDB_OPTIONS=--sgml-mode --output-format=xml

-- 
1.7.1