contribs: png: disable debug lib suffix for all platforms
requested to merge umxprime/vlc:umxprime/4.x/contribs/png/remove-library-debug-suffix/mr into master
Given contribs are bootstrapped with --disable-optim option,
the CMAKE flag -DCMAKE_BUILD_TYPE is set to Debug.
When build configuration is set to Debug in libpng contrib, it CMAKE
configuration append a d char to the static library name,
resulting in a lib named libpng16d.a instead of libpng16.a.
Then some vlc module depending on libpng aren't able to link
properly as they can't find their -lpng16 dependency.