LU-4085 build: gss/krb5 is disabled despite functions found

The function available check for krb5int_derive_key and
krb5_derive_key by means of AC_CHECK_LIB($gssapi_lib,...)
does not set shell variables HAVE_KRB5_DERIVE_KEY,
HAVE_KRB5INT_DERIVE_KEY so the test
AS_IF([test "x$HAVE_KRB5INT_DERIVE_KEY" = "x1"
-o "x$HAVE_KRB5_DERIVE_KEY" = "x1"] gives false despite
the functions are found. As a side effect this fix
revealed that calls with real prototypes are required.
Thus prototypes of krb5_derive_key(...) and
krb5int_derive_key(...) are provided, and required
structures from krb5-int.h are copied.

Signed-off-by: Thomas Stibor <thomas@stibor.net>
Change-Id: I8c4cce5f861c9e4d01071024c4b852a2274c1b40