Currently we have $BUILD_DIR which is actually the VariantDir, and $BUILD_ROOT which is the argument to --build-dir. We also have $INSTALL_DIR but a flag called --prefix. Finally, we are going to be adding a new --dest-dir flag and it needs a new associated variable.

We should normalize these as follows:

• Rename $BUILD_DIR to $VARIANT_DIR
• Rename $BUILD_ROOT to $BUILD_DIR