EDIT 2: Despite being called "GNUmakefile" those makefiles do not even respect DESTDIR, so ignore that part. And it seems "prefix" is really just used almost how DESTDIR should be and is never actually used the way the common makefile "prefix" variable is - so including pkgdir in it should be ok ... but it still makes me cringe having that in a PKGBUILD. (it's safe as "prefix" is never passed as a macro definition in a compiler command, but it's only used for 'cp' commands ... yes 'cp', not 'install').