Problem: configure.ac is not setting HAVE_LIBGSSAPI_KRB5
in src/platform.hpp when --with-libgssapi_krb5 is specified

Commit 09e868b7
switched the libgssapi_krb5 check from AC_CHECK_LIB
to AC_SEARCH_LIBS, but neglected to add an AC_DEFINE
for HAVE_LIBGSSAPI_KRB5, thus the GSSAPI code is
never compiled.

Solution: Add missing AC_DEFINE of HAVE_LIBGSSAPI_KRB5.