audacity-2.1.0 and libpng

On linux (slackware64-current) I have compiled audacity-2.1.0 with libpng-1.6.16. Compilation succeeds, but no icons appear when the program is run : I have a message telling that the default theme cannot be loaded. The problem disappears when audacity is compiled using libpng-1.4.12.

I expect that 1.6.16 is much newer than any of the developers are using. Even Ubuntu, which tends to have more recent packages than most distros uses 1.2.51.
The Audacity developers have recently updated a load of libraries, but I don’t think that libpng was among them.