config.mk: Use prefix in ICON_PREFIX

[avindra]

The PREFIX, if specified, should also be used to determine the ICON_PREFIX

This was originally written by @mimi1vx who I've tagged here.

[derf]

PREFIX is deliberately ignored in this case.

make install app=1 (the only case in which this branch is relevant) is meant for end users installing feh via the Makefile on their system, so PREFIX is /usr/local in most cases. However, icons in /usr/local/share/icons are not generally supported, so even though feh itself ends up in /usr/local, its icons must be installed into /usr/share/icons.

So it may look like an omission, but is intentional. Thanks for taking the time to submit a fix, though :)