From june 2023:
http://www.libpng.org/pub/png/src/libpng-1.6.40-README.txt

 * Fixed the eXIf chunk multiplicity checks.
 * Fixed a memory leak in pCAL processing.
 * Corrected the validity report about tRNS inside png_get_valid().
 * Fixed various build issues on *BSD, Mac and Windows.
 * Updated the configurations and the scripts for continuous integration.
 * Cleaned up the code, the build scripts, and the documentation.

100% amd64 tests pass, my Xfce4 desktop is happy.
No WANTLIB, symbol or PLIST change.

Feedback? OK?

Index: Makefile
===================================================================
RCS file: /cvs/ports/graphics/png/Makefile,v
diff -u -p -r1.131 Makefile
--- Makefile	26 Sep 2023 11:39:09 -0000	1.131
+++ Makefile	9 Jan 2024 22:58:57 -0000
@@ -1,6 +1,6 @@
 COMMENT=	library for manipulating PNG images
 
-VERSION=	1.6.39
+VERSION=	1.6.40
 DISTNAME=	libpng-${VERSION}
 PKGNAME=	png-${VERSION}
 CATEGORIES=	graphics
Index: distinfo
===================================================================
RCS file: /cvs/ports/graphics/png/distinfo,v
diff -u -p -r1.62 distinfo
--- distinfo	12 Jan 2023 19:19:49 -0000	1.62
+++ distinfo	9 Jan 2024 22:59:09 -0000
@@ -1,2 +1,2 @@
-SHA256 (libpng-1.6.39.tar.xz) = H0aWznC07l+F8eFiPcEimyEAKfpLeu5XPfPiunsDaTc=
-SIZE (libpng-1.6.39.tar.xz) = 1020552
+SHA256 (libpng-1.6.40.tar.xz) = U1tHmyRn/yMaPsbZKlJZBvuO8nl4vk9m2+BdPzoBs6E=
+SIZE (libpng-1.6.40.tar.xz) = 1021332