Add configure.sysroot variable
Add configure.sysroot variable

This is based on configure.sdkroot and contains either the path to the
SDK, or '/' if no SDK is in use, so it can be used in contexts where a
valid filesystem path where system headers and libraries can be found
is always needed.

Fixes: https://trac.macports.org/ticket/59798

neverpanic/macports-base,danchr/macports-base,macports/macports-base,neverpanic/macports-base,danchr/macports-base,macports/macports-base