Fix build issues with Solaris native compiler

Fix build issues with Solaris native compiler

In the LDAP KDB module, fix an empty initializer.  In the SPAKE
edwards25519 code, use autoconf tests to determine whether to use the
64-bit code.  In the SPAKE update_thash() function, make sure the
types of the conditional expression results match exactly.  In
libkrb5support, link against zap.o now that k5buf.o can use zap() (as
of commit 8ee8246c14702dc03b02e31b9fb5b7c2bb674bfb).

[ghudson@mit.edu: squashed commits; rewrote commit message; adjusted
autoconf tests; minor code changes]

(cherry picked from commit ecb03a4c31cf8a6b1bca3459ae56d4122398c18e)

ticket: 8769
version_fixed: 1.17