Support overriding the prefix and DESTDIR variables from the make command
line. This makes the makefile system a little more GNU compliant and the
feature makes it easier for distribution builders to re-target the install
location of LLVM.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@33305 91177308-0d34-0410-b5e6-96231b3b80d8