https://bugs.gentoo.org/955661
https://github.com/opencv/opencv/pull/27314

From 8fae4a65feb334de530174fd17147637c241e6b8 Mon Sep 17 00:00:00 2001
From: Alfred Wingate <parona@protonmail.com>
Date: Wed, 14 May 2025 23:30:10 +0300
Subject: [PATCH] cmake: set SPNG_LIBRARY for pkgconfig as well

Pkgconfig will set SPNG_LIBRARIES but not SPNG_LIBRARY, this is an issue
as modules/imgcodecs/CmakeLists.txt uses SPNG_LIBRARY.

Bug: https://bugs.gentoo.org/955661
Fixes: c92815238e083563f1b03ac8c449ed848b13b0d9
Signed-off-by: Alfred Wingate <parona@protonmail.com>
--- a/cmake/OpenCVFindLibsGrfmt.cmake
+++ b/cmake/OpenCVFindLibsGrfmt.cmake
@@ -304,6 +304,9 @@ if(WITH_SPNG)
     else()
       if(PkgConfig_FOUND)
         pkg_check_modules(SPNG QUIET spng)
+        if(SPNG_FOUND)
+          set(SPNG_LIBRARY ${SPNG_LIBRARIES} CACHE INTERNAL "")
+        endif()
       endif()
     endif()
     if(SPNG_FOUND)
-- 
2.49.0