This fixes cross-compiling from Linux to Android. My guess is that the
build system nowadays, when building the cross_toolset (for the build
platform), sources Env.Build.sh in an environment where Env.Host.sh
has already been sourced. So, if Env.Host.sh exports SYSBASE, that
stayed in force also after sourcing Env.Build.sh, with predictably
horrible results.

For some reason it didn't hurt cross-compiling from Mac OS X, which is
why I hadn't noticed.