pkg.use-bootstrap can now be set to true to add a bootstrap phase to
cross-builds. I.e. the package is built for the native platform and
installed to a temporary location, which is in the PATH during the
actual cross-compilation. This feature is useful for some misbehaving
ports that can cross-compile, but require the exact same version of the
software installed locally. The bootstrap build is controlled with the
bootstrap.foo variables rather than the normal pkg.foo variables.

pkg.source-package can now be set to the name of another package, whose
source code is built using the current tixbuildinfo. This feature allows
providing multiple packages using the same source code package. By
default, the source code of the source package is assumed to be in
../${pkg.source-package}, but this can be overridden with the option
--source-directory.

pkg.alias-of can now be set to the name of another package to specify
that this package is an alias of the other package, creating an empty
binary package depending on the real package.

pkg.subdir support has been fixed in the clean and post-install phases.

pkg-config support has been improved and PKG_CONFIG is now set to
$HOST-pkg-config and PKG_CONFIG_FOR_BUILD is set to pkg-config.

tix-build has been refactored as needed and generally cleaned up. Error
handling, such as on allocations, have been added in a lot of cases. The
support for FOO_FOR_BUILD variables have been unified and simplified.
Appending to PATH now correctly handles the empty PATH.