Respect host tool prefixes for krb5-config

Currently, curl unconditionally uses `krb5-config`. This breaks multilib support on Gentoo which requires you to use `i686-pc-linux-gnu-krb5-config` when building 32-bit version of curl. It may also cause trouble with some scenarios of cross-compilation.

Support for this is usually handled via the `AC_PATH_TOOL` autoconf macro that prefers `$host_alias-krb5-config` over `krb5-config` if available. Since curl doesn't use standard autoconf macros, I inlined the same logic `AC_PATH_TOOL` uses into the current check. Since it has a fallback to regular `krb5-config`, it doesn't cause trouble for other systems.