Allow inst_dir to be set with -B/-b.

This changes the tar_em_up.sh script so that it becomes possible to set the installation prefix via the inst_dir environment variable even when the -B or -b option is used (previously these options would always override the value of inst_dir to become /usr). The default behavior of these (as well as any other) build options is unaffected.

I need this in order to build Linux packages of purr-data which can be installed alongside pd-l2ork (as well as other Pd flavors), as was requested on the l2ork-dev mailing list (cf. https://bitbucket.org/l2orkubuntu/l2orkubuntu.bitbucket.org/issues/1/package-bug).