Revert "build: fix `make install DESTDIR=/path`"

This reverts commit f8051397.

The reverted commit made DESTDIR behave more like we want it to but it was
pointed out in #3489 that it makes life a lot harder for distro package
maintainers.

Those guys and gals already have a hard enough time as it is, let's not make
their jobs even more hellish.