[latexila] build: set AM_VALAFLAGS instead of VALAFLAGS

[Sébastien Wilmet <swilmet src gnome org>]

commit 15cde3098527648adeb6bbb52320b7011b2207b9
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sat Mar 11 14:56:17 2017 +0100

    build: set AM_VALAFLAGS instead of VALAFLAGS
    
    The build warning:
    src/Makefile.am:7: warning: 'VALAFLAGS' is a user variable, you should not override it;
    src/Makefile.am:7: use 'AM_VALAFLAGS' instead

 src/Makefile.am |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/Makefile.am b/src/Makefile.am
index ee1821f..5eee1e8 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -4,7 +4,7 @@ bin_PROGRAMS = latexila
 
 # TODO remember to remove --disable-since-check when GtkSourceView 3.24 is
 # released...
-VALAFLAGS =                    \
+AM_VALAFLAGS =                 \
        --pkg gtk+-3.0          \
        --disable-since-check   \
        --pkg gtksourceview-3.0 \