Various build fixes, mostly uncovered by clang's unused-const-variable and unused-function errors. Reviewed-by: Kurt Roeckx <kurt@openssl.org> (cherry picked from commit 0e1c318ece3c82e96ae95a34a1badf58198d6b28)
Also, use the common Configure mechanism for enabling/disabling the 64-bit ECC code.
NIST-P521. (Now -DEC_NISTP_64_GCC_128 enables all three of these; -DEC_NISTP224_64_GCC_128 no longer works.) Submitted by: Google Inc.