staging-prefix not respected by cmake files

[rossburton]

If I install Boost with b2 install --prefix=/usr staging-prefix=/tmp/stage/usr the libraries are installed to /tmp/stage/usr/lib/... but fails as the the cmake files are not relocated under the staging prefix:

common.mkdir /usr/lib/cmake
| mkdir: cannot create directory ‘/usr/lib/cmake’: Permission denied

[pdimov]

staging-prefix is a new feature, added in boostorg/build@51ad471. It's not clear how these new features are supposed to interact with the old command-line options (--prefix, --libdir etc) used when installing Boost.

[pdimov]

Other than that, the functionality has already been requested in the form of DESTDIR support, which I was going to implement as a --destdir option, but --staging-prefix works too, I guess. #34

[rossburton]

My impression was that staging-prefix is basically DESTDIR, but if you're going to add proper DESTDIR semantics then that would be better.