We must be able to use a non-native strip executable, but not all
versions of 'install' support the --strip-program option (e.g.
OpenBSD). Accordingly, we can't use 'install -s', and we must run strip
separately.

Signed-off-by: Hollis Blanchard <hollis@penguinppc.org>
Cc: blauwirbel@gmail.com