[CMake] "sudo make install; DESTDIR=foo make install" fails

[Dan Kegel]

If "sudo make install" is done in btmp, it leaves a file called
"install_manifest.txt" in the btmp directory owned by root. This causes a later
non-sudo "make install" (where configuration has changed the destination
directory to something that does not require sudo) to fail.

Perhaps "make install" should remove install_manifest.txt in that case
rather than failing?