New define WITH_PUBLIC_KEY, and new configure flag

--disable-public-key. Updated rsa-files to check for that, rather
than for HAVE_LIBGMP.

Rev: src/nettle/rsa-keygen.c:1.3
Rev: src/nettle/rsa.c:1.8
Rev: src/nettle/rsa2sexp.c:1.2
Rev: src/nettle/rsa_md5.c:1.5
Rev: src/nettle/rsa_sha1.c:1.5
Rev: src/nettle/sexp2rsa.c:1.3
Rev: src/nettle/testsuite/rsa-keygen-test.c:1.4
Rev: src/nettle/testsuite/rsa-test.c:1.5
Rev: src/nettle/testsuite/rsa2sexp-test.c:1.2
Rev: src/nettle/testsuite/sexp2rsa-test.c:1.2
Rev: src/nettle/testsuite/testutils.c:1.13
Rev: src/nettle/testsuite/testutils.h:1.11