build: allow installing into the root directory (/)

It turns out that CMake always canonicalizes `CMAKE_INSTALL_PREFIX` to
an absolute path--if it's a relative path, it canonicalizes it relative
to the build directory.  As a result, the only thing the DESTDIR and
relative directory check prevents is an installation into the root
directory since CMake strips the trailing slash, turning "/" into an
empty string.  Let's just remove the check all together, since it cannot
accomplish what we intended.