cryptsetup: fix build with old host kernel headers

cryptsetup: fix build with old host kernel headers

By default cryptsetup configure fails when the if_alg.h kernel header is
not available. Kernels older than 2.6.38, like the one in RHEL 6 hosts,
do not provide this header. Since we don't need the kernel crypto
feature for host tools, just disable this feature to allow successful
completion of the configure script.

While at it, fix a typo in the --with-crypto_backend option (hyphen ->
underscore).

Should fix:
http://autobuild.buildroot.net/results/ffd/ffda2579b215b53161025b7bc703091cb30ef95d/
http://autobuild.buildroot.net/results/c48/c48015f76b13fec38f650f66824e7b986eb28572/
http://autobuild.buildroot.net/results/be6/be621f71bf9da8719bf7fc943eccc9b4dc2cc43e/

Cc: Martin Hicks <mort@bork.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>