Support DESTDIR on installation
Support DESTDIR on installation

Most build systems allow a DESTDIR environment variable, for staging an installation from one location to another. For example, if Mono is in `/usr/lib/mono` but you want to copy the installation files to a temporary staging directory of `/tmp/foo`, `DESTDIR=/tmp/foo ./install-mono-prefix.sh /usr` will install the files to `/tmp/foo` but symlinks and modifications to `bin/msbuild` will look for Mono etc in `/usr`, not `/tmp/foo`

mono/msbuild,mono/msbuild,mono/msbuild,mono/msbuild