top-level configure, but to ${MAKE} configure-host as well.
Add more vars (for readline mostly) so that there's no configure
warnings about tests skipped b/c of cross-compilation.
gdb comes with its own copy of GNU readline 5.x, but
-I${DESTDIR}/usr/include stuffed at the beginning of the command line
makes it find our compat readline/readline.h (emulating readline 2.x)
that conflicts with the readline 5.x includes.
Makes README.mknative instructions actually work instead of failing
midway b/c of conflicting tilde_expand() prototypes.