Android hints: set targetsh to /system/bin/sh

Android hints: set targetsh to /system/bin/sh

Android, for reasons best known to itself, doesn't have a "/bin/sh" it
has a "/system/bin/sh".  We need to run "/bin/sh" as $sh inside
./Configure on the host, but store a different path for use on the
actual Android system (eg by backticks, system etc, in SH_PATH).

The new variable is "targetsh" and defaults to "sh" unless changed by the hints file/Configure params.