With this patch the make install target for Noux applications gets defined
in the 'NOUX_INSTALL_TARGET' make variable with 'install-strip' as default.
Fixes#503.
The directory specified by --prefix is not only used to determine
the install location but is also used by -rpath. Therefor it is
better to use $(DESTDIR) to install the binaries to the right
directory and to use --prefix to express the actual location within
NOUX.