[glib] docs: Ensure CLEANFILES is set before we use +=

[Colin Walters <walters src gnome org>]

commit ac7c050d377f6815534074b855cdf74f02cf96b2
Author: Colin Walters <walters verbum org>
Date:   Sat Aug 25 17:13:43 2012 -0400

    docs: Ensure CLEANFILES is set before we use +=
    
    We need to be defensive about this in the case where gtk-doc.make is
    empty.

 docs/reference/gio/Makefile.am     |    1 +
 docs/reference/glib/Makefile.am    |    1 +
 docs/reference/gobject/Makefile.am |    1 +
 3 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gio/Makefile.am b/docs/reference/gio/Makefile.am
index 4167069..64d95fe 100644
--- a/docs/reference/gio/Makefile.am
+++ b/docs/reference/gio/Makefile.am
@@ -168,6 +168,7 @@ XSLTPROC_FLAGS = \
 
 endif
 
+CLEANFILES ?=
 CLEANFILES += $(man_MANS)
 
 EXTRA_DIST += $(man_MANS)
diff --git a/docs/reference/glib/Makefile.am b/docs/reference/glib/Makefile.am
index 0ca622e..88404ed 100644
--- a/docs/reference/glib/Makefile.am
+++ b/docs/reference/glib/Makefile.am
@@ -116,6 +116,7 @@ XSLTPROC_FLAGS = \
 
 endif
 
+CLEANFILES ?=
 CLEANFILES += $(man_MANS)
 
 EXTRA_DIST += $(man_MANS)
diff --git a/docs/reference/gobject/Makefile.am b/docs/reference/gobject/Makefile.am
index 051883a..48d88f8 100644
--- a/docs/reference/gobject/Makefile.am
+++ b/docs/reference/gobject/Makefile.am
@@ -92,6 +92,7 @@ XSLTPROC_FLAGS = \
 
 endif
 
+CLEANFILES ?=
 CLEANFILES += $(man_MANS)
 
 EXTRA_DIST += $(man_MANS)