[libpng] usage as indicated in usage file may result in system library being prefered over vcpkg version

[ccptoebeans]

Is your feature request related to a problem? Please describe.

https://github.com/microsoft/vcpkg/blob/master/ports/libpng/usage

libpng provides CMake targets:

  find_package(PNG REQUIRED)
  target_link_libraries(main PRIVATE PNG::PNG)

The above usage information for libpng is risky. If the user has a local PNG installation, cmakes FindPNG module will prefer that package over the VCPKG one.

Proposed solution

Usage must be changed to:

libpng provides CMake targets:

  find_package(libpng CONFIG REQUIRED)
  target_link_libraries(main PRIVATE PNG::PNG)

[Osyotr]

If the user has a local PNG installation, cmakes FindPNG module will prefer that package over the VCPKG one.

How come?

[dg0yt]

The above usage information for libpng is risky. If the user has a local PNG installation, cmakes FindPNG module will prefer that package over the VCPKG one.

It is not more risky than using the CMake config.
FindPNG will not prefer the system installation when using the vcpkg CMake toolchain properly. This must work, or PNG as a transitive dependency would be broken.