[gimp] configure.ac: require libpng 1.6.25

[Michael Natterer <mitch src gnome org>]

commit a5073ad9289f7e05ef38d2ce99dc71e91983b434
Author: Michael Natterer <mitch gimp org>
Date:   Mon Oct 31 09:36:15 2016 +0100

    configure.ac: require libpng 1.6.25
    
    Addresses bug #765850, and probably avoids a gazillion of
    vulnerabilities in the versions since the ancient 1.2.37
    we required before.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 34e8dc2..c486394 100644
--- a/configure.ac
+++ b/configure.ac
@@ -69,7 +69,7 @@ m4_define([libgudev_required_version], [167])
 m4_define([gexiv2_required_version], [0.6.1])
 m4_define([libmypaint_required_version], [1.3.0])
 m4_define([lcms_required_version], [2.7])
-m4_define([libpng_required_version], [1.2.37])
+m4_define([libpng_required_version], [1.6.25])
 m4_define([liblzma_required_version], [5.0.0])
 m4_define([openexr_required_version], [1.6.1])
 m4_define([gtk_mac_integration_required_version], [2.0.0])