Don't use WITH_PUBLIC_KEY / WITH_HOGWEED, then Makefile sorts out

which files should be compiled.

Rev: nettle/der2rsa.c:1.2
Rev: nettle/dsa-keygen.c:1.2
Rev: nettle/dsa-sign.c:1.2
Rev: nettle/dsa-verify.c:1.2
Rev: nettle/dsa.c:1.2
Rev: nettle/pkcs1-rsa-md5.c:1.2
Rev: nettle/pkcs1-rsa-sha1.c:1.2
Rev: nettle/pkcs1-rsa-sha256.c:1.2
Rev: nettle/pkcs1.c:1.2
Rev: nettle/rsa-compat.c:1.2
Rev: nettle/rsa-decrypt.c:1.2
Rev: nettle/rsa-encrypt.c:1.2
Rev: nettle/rsa-keygen.c:1.2
Rev: nettle/rsa-md5-sign.c:1.2
Rev: nettle/rsa-md5-verify.c:1.2
Rev: nettle/rsa-sha1-sign.c:1.2
Rev: nettle/rsa-sha1-verify.c:1.2
Rev: nettle/rsa-sha256-sign.c:1.2
Rev: nettle/rsa-sha256-verify.c:1.2
Rev: nettle/rsa-sign.c:1.2
Rev: nettle/rsa-verify.c:1.2
Rev: nettle/rsa.c:1.2
Rev: nettle/rsa2openpgp.c:1.2
Rev: nettle/rsa2sexp.c:1.2
Rev: nettle/sexp2dsa.c:1.2
Rev: nettle/sexp2rsa.c:1.2