openssl: use --cross-compile-prefix in Configure

openssl: use --cross-compile-prefix in Configure

This sets the --cross-compile-prefix option when running Configure, so
that that it will not use the host gcc to figure out, among other
things, compiler defines.  It avoids errors, if the host 'gcc' is
handled by clang:

mips-openwrt-linux-musl-gcc: error: unrecognized command-line option
'-Qunused-arguments'

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>
Tested-by: Rosen Penev <rosenp@gmail.com>