[gdk-pixbuf] build: fix undefined os_win32 var on macOS

[Christoph Reiter <creiter src gnome org>]

commit 24794b086cc4b7f78d4a45786d66368b44ffc8de
Author: Christoph Reiter <creiter src gnome org>
Date:   Sun Sep 17 19:42:59 2017 +0200

    build: fix undefined os_win32 var on macOS
    
    In 4c63e6fea6ceda96af72c9 a new case was added without defining
    os_win32 breaking some subsequent condition checks.
    
    Thanks to Rainer Müller for spotting the mistake.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=786167

 configure.ac |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index c07bd6f..719daf4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -117,6 +117,7 @@ case "$host" in
     esac
     ;;
   *-*-darwin*)
+    os_win32=no
     gio_can_sniff=no
     ;;
   *)