Use OPEN_DYLAN_PLATFORM_NAME to set LIB dirs.

OPEN_DYLAN_PLATFORM_NAME is available to JAM as TARGET_PLATFORM.

We were using OS and OSPLAT, but using OPEN_DYLAN_PLATFORM_NAME
will make things more consistent and work with things like
emscripten and the Java backend. It also makes cross-compilation
work again by just setting OPEN_DYLAN_PLATFORM_NAME and everything
else working from that.
Export target-platform-name, re-use it.
Rename OPEN_DYLAN_PLATFORM_NAME to OPEN_DYLAN_TARGET_PLATFORM.
Release notes for OPEN_DYLAN_TARGET_PLATFORM.
Merge pull request #505 from waywardmonkeys/cross-build-fixes

Use OPEN_DYLAN_PLATFORM_NAME to set LIB dirs.