cmake: explicitly define PNG_WRITE_APNG_SUPPORTED if using vcpkg

Author: adriweb

gui/qt/CMakeLists.txt

@@ -287,6 +287,10 @@ find_package(PNG QUIET)
 if(PNG_FOUND)
     target_compile_definitions(CEmu PRIVATE "PNG_SUPPORT")
     target_link_libraries(CEmu PRIVATE PNG::PNG)
+    # vcpkg provides a libpng that handles APNG
+    if(VCPKG_TARGET_TRIPLET)
+        target_compile_definitions(CEmu PRIVATE "PNG_WRITE_APNG_SUPPORTED")
+    endif()
 else()
     message(WARNING "No LibPNG found! APNG capture will not be available")
 endif()