build: Fix make install for cross builds.

build: Fix make install for cross builds.

make install discards symbols from executables by using the -s option
to the install command. If no install command was detected at configure
time, the install-sh script shipped in the paraslash source tree is
used instead.

This is currently broken for cross-builds because install -s runs the
native strip command by default but the cross-strip is required here.

Both the install script and the install command from gnu coreutils
support custom strip commands, but the syntax for specifying these are
different. A decent install program is available at least on FreeBSD,
NetBSD, MacOS and Linux, so let's just kill the script and use the
--strip-command option of the install program to allow the user to
specify a suitable cross-strip.