pngquant on head-amd64-default-ssp

head-amd64-default-ssp:
[..]
===>   pngquant-2.0.0 depends on shared library: libpng15.so - not found
===>    Verifying for libpng15.so in /usr/ports/graphics/png
===>   Installing existing package /packages/All/png-1.5.18.txz
Installing png-1.5.18... done
[..]
a - libimagequant.o
gmake[2]: Leaving directory `/wrkdirs/usr/ports/graphics/pngquant/work/pngquant-2.0.0/lib'
cc pngquant.o rwpng.o   -fstack-protector -lpng -lz -lm lib/libimagequant.a -lm    -o pngquant
/usr/bin/ld: cannot find -lpng
cc: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[1]: *** [pngquant] Error 1
gmake[1]: Leaving directory `/wrkdirs/usr/ports/graphics/pngquant/work/pngquant-2.0.0'
*** Error code 1