make distdir-am fails when png2icns is not installed, with confusing message

[apoelstra]

make distdir-am
make -C src qt/res/rendered_icons/bitcoin.ico
make[1]: Entering directory '/home/apoelstra/code/ElementsProject/elements/master/src'
make[1]: 'qt/res/rendered_icons/bitcoin.ico' is up to date.
make[1]: Leaving directory '/home/apoelstra/code/ElementsProject/elements/master/src'
make -C src qt/res/rendered_icons/nsis-wizard.bmp
make[1]: Entering directory '/home/apoelstra/code/ElementsProject/elements/master/src'
make[1]: 'qt/res/rendered_icons/nsis-wizard.bmp' is up to date.
make[1]: Leaving directory '/home/apoelstra/code/ElementsProject/elements/master/src'
make -C src qt/res/rendered_icons/bitcoin.icns
make[1]: Entering directory '/home/apoelstra/code/ElementsProject/elements/master/src'
no qt/res/rendered_icons/bitcoin.icns qt/res/rendered_icons/bitcoin256.png qt/res/rendered_icons/bitcoin512.png qt/res/rendered_icons/bitcoin1024.png qt/res/rendered_icons/bitcoin32.png qt/res/rendered_icons/bitcoin16.png
make[1]: no: No such file or directory
make[1]: *** [Makefile:15146: qt/res/rendered_icons/bitcoin.icns] Error 127
make[1]: Leaving directory '/home/apoelstra/code/ElementsProject/elements/master/src'
make: *** [Makefile:1243: src/qt/res/rendered_icons/bitcoin.icns] Error 2

It's because configure.ac is set to look for png2icns, setting the PNG2ICNS variable to no if it's not found, which we just blindly try to use as a binary name.

Introduced in #569

[apoelstra]

Closing. png2icns was removed in a1bac6d which was part of #935